ST? 

'5? 



NATURAL HISTORY AND PROPAGATION OF 
FRESH-WATER MUSSELS :::::::: 
By R. E. Coker, A. F. Shira, H. W. Clark, and A. D. Howard 

From BULLETIN OF THE BUREAU OF FISHERIES, Volume XXXVII, 1919-20 
Document No. 893 : : : : : : : : : : : : : : : Issued May 2, igsi 




PRICE, 25 CENTS 
Sold onlir by the Superintendent of Documents, Government Printing Office, Wasliington, D. C. 



WASHINGTON :::::: GOVERNMENT PRINTING OFFICE 



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/^ 



NATURAL HISTORY AND PROPAGATION OF 
FRESH-WATER MUSSELS :::::::: 
By R. E. Coker, A. F. Shira, H. W. Clark, and A. D. Howard 

From BULLETIN OF THE BUREAU OF FISHERIES, Volume XXXVII, 1919-20 
Document No. 8gj : : : : : : : : : : : : : : : Issued May 2, ig2i 




PRICE, 25 CENTS 
Sold only by the Superintendent of Documents. Government Printing Office, Washington, D. C. 



WASHINGTON : 



: GOVERNMENT PRINTING OFFICE : 



; 1921 



6< 



-UrfTiWili — -~ 



LIBRARY OF CQNQRESS 

MAY 6-1921 

DOCUMBNT& LiiV.uklON 



A 



CONTENTS. 

Page. 

Introduction 79 

Part I . Natural history of fresh- water mussels 81 

Habits 81 

Conditions of existence 81 

Locomotion 82 

Density of population 84 

Breeding 85 

Winter habits 85 

Feeding habits 86 

Food of mussels 87 

Significance of the problem 87 

Observations of Franz Schrader on food of mussels 88 

Species studied 88 

Food content of waters 88 

Food discrimination under normal conditions i'. 89 

Utilization of food materials ;........ 89 

Experiments in feeding vegetable matter 89 

Experiments in feeding animal matter 90 

General observations 90 

Observations of H. Walton Clark on food of mussels 91 

Observations of A. F. Shira on food of juvenile mussels 93 

Habitat 94 

Body of water 94 

Streams 94 

I<akes 97 

Ponds, sloughs, marshes, and swamps lOo 

Artificial ponds and canals 100 

Bottom loi 

Depth 107 

Light no 

Current in 

Water content 113 

Suspended matter 1 14 

Minerals in solution 114 

Dissolved gases 117 

Vegetation 118 

Animal associates 119 

Parasites and enemies 121 

Parasites 121 

Enemies 122 

Conditions unfavorable for mussels 123 

Natiural conditions 123 

Artificial conditions 124 

Growth and fonnation of shell 125 

Measurements of gro\vth 125 

Presence of so-called growth rings 129 

Mode of fonnation of shell 129 

Significance of rings 132 

Abnormalities in growth of shell 133 

77 



78 CONTENTS. 

Page. 

Part 2. Life history and propagation of fresh-water mussels 135 

Introduction ^ 135 

Historical note 136 

Age at which breeding begins 137 

Ovulation and fertilization 138 

Brood pouches or marsupia 138 

Seasons of deposition of eggs 140 

Seasons of incubation of eggs 140 

Glochidium 143 

Stage of parasitism 148 

Hosts of fresh-water mussels 151 

Parasitism and immimit}- 155 

Metamorphosis without parasitism 156 

Juvenile stage 157 

Artificial propagation 160 

Principle of operation 1 60 

Methods 160 

Mussel culture 1 63 

Part 3. Structure of fresh-water mussels 1O7 

Introduction 167 

The shell 168 

External features i68 

Internal features 169 

DiversitTi' in form 1 70 

The soft body 171 

Form and functions of the mantle 171 

Other conspicuous organs 172 

Internal structure 173 

Stmctiu'e and functions of the gills 174 

Bibliography 177 



NATURAL HISTORY AND PROPAGATION OF FRESH-WATER 

MUSSELS. 

By 

R. E. COKER, Assistant in Charge Scientific Inquiry, 

A. F. SHIRA, Lately Director Fisheries Biological Station, Fairport, Iowa, 

H. WALTON CLARK, Scientific Assistant, and 

A. D. HOWARD, Scientific Assistant. 

INTRODUCTION. 

Adult fresh-water mussels are free-living but sedentary in habit. Though attached 
to nothing they remain for indefinite periods nearly as still as if their position were irre- 
vocably fixed. They have powers of locomotion but only occasionally use them. A 
snail is expected to be in travel, however slow, in the search for food; but when a mussel 
is found in motion the observer is inclined to look for a special cause of this behavior. 

If a living animal remains generally in one place without going after its food, it 
must have some effective mechanism for bringing food to itself, and it must also depend 
in part upon outside agencies to convey its food within reach. In the fresh-water mus- 
sel, the mechanism employed for food gathering consists of hundreds of thousands of 
active microscopic paddles covering the flaps that hang from the side of its body. These 
paddles are exceedingly minute and all within the shell; each is weak and ineffective 
alone, but the effect of their concurrent action is to keep a strong current of water passing 
into the mussel and out again. The water is filtered in passing, and the food, of course, 
consists of the fine materials suspended and perhaps in part dissolved in the water. 
The food in the water that lies within the influence of its currents is thus available to 
the mussels; the natural circulation of the outside water must do the rest. 

A single animal that finds food brought within its reach might live the full period 
of its life in one spot, but all animals of a species can not live in the same spot. It is 
inevitable, therefore, that at some stage in the life history of such an animal as the 
fresh-water mussel, there must be a period of movement or of distribution by outside 
agencies. Through one of nature's nice adaptations, such a period of migration or dis- 
tribution occurs in the life history of the mussel at the stage of infancy. Even then the 
mussel shifts the burden of its distribution upon fish, as will be more fully told in the 
section on life history. As inactive in youth as in old age, the fresh-water mussel, hav- 
ing taken passage upon a fish, may then travel extensively to find a new home far removed 
from the scene of its birth. Its living conveyance dispensed with, the mussel settles 
down to a relatively immobile existence. 

Peculiarly victims of circumstance at all stages of existence, the fresh-water mus- 
sels under natural conditions yet throve abundantly and broadly in streams and lakes 

79 



8o BULLETIN OF THE BUREAU OF FISHERIES. 

of this country and in those of other countries, but more especially in the Mississippi and 
Great Lakes drainages. For them, however, times changed with the discovery that their 
shells formed a good material for the manufacture of a universal necessity — buttons. 
Equal as they were to the vicissitudes of natural conditions, they were unable to with- 
stand the unchecked ravages of commercial fishery. Thus there has arisen the necessity 
for measures of conservation— propagation and protection. 

It is the purpose of the present report to present such an account of the structure 
and habits and relations of fresh-water mussels as will serve to diffuse knowledge of 
fresh-water mussels and interest in them, as may promote intelligent measures for their 
conservation and efficiency in propagation and as may stimulate investigation of the many 
problems presented by thebehavior, distribution, and propagation of mussels. Directed as 
it is both to the layman and to the scientist, the report must labor under the disadvantage 
of embodying matter that may seem trite to the scientist and much that may seem 
overtechnical to the layman. As far as possible, however, the more technical data are 
omitted or embodied in tables which can be passed over by those who are not interested 
in the details. 

The first part, on the natural history of mussels or the relation to their environments, 
embodies data from many sources, but more especially from the general experience of 
the several authors. In the second part is comprised perhaps the greatest measure of 
original data gained from experiments and investigations conducted at the U. S. Fish- 
eries Biological Station, Fairport, Iowa, though there are incorporated also results of 
the published investigations of Lefevre and Curtis and of others. The third part, pre- 
senting a summarized account of the structure, does not pretend to offer new data, but 
rather to afford a background of knowledge of the mussel as a complex living animal 
with many functions and needs. It might have been placed first but that it seemed best 
to begin with the subjects which constitute the essential purpose of the paper. 



Bull. U. vS. B. F., ioio-2c 



Plate Y 





Fig. 1. — Rear view of yellow sand-shell in natural position 
in bottom under water, showing the two siphonal open- 
ings, the gaping shell, and the apposed margins of the 
mantle. 



Fig. 2. — Rear view of Anodonta corPulenta showing 
siphonal openings. Note the very smooth margins of 
the upper exhalent opening and the fimbriae or "feel- 
ers" protecting the lower or inhalent opening. 




-Tracks of >ouug mussels iu crate 



Change in the conditions uf the water had caused them to ' 
normally. 



audcr more than 



PART 1. NATURAL HISTORY OF FRESH- WATER MUSSELS. 

HABITS. 

CONDITIONS OF EXISTENCE. 

A mussel, in natural position in a stream, is partly or almost entirely embedded 
in the sand, mud, or gravel of the bottom (PI. V, fig. i). Almost invariably it will be 
found to have an oblique position, the front end of the body being directed down into 
the bottom and in a direction with the flow of the current, while the hinder end of the 
shell is exposed and is directed upward and against the flow of the stream. Unless the 
mussel has been disturbed, the shell will be slightly gaping, with the edges of the mantle 
protruding through the opening and closing it everywhere except at the rear (upper) 
end where it is so arranged as to form two neat funnel-like openings. The upper open- 
ing is usually the smaller, and the edges of the mantle about it are smooth or crinkled. 
The lower opening is generally much longer, and the border of the mantle here is com- 
monly adorned with a number of delicate feelers, or water testers as these may be called 
(PI. V, figs. I and 2). The significance of the two openings can be easily ascertained 
if a small amount of some colored liquid, such as finely powdered carmine in water, is 
placed near to the openings in a mussel which has been allowed to remain undisturbed 
in a small aquarium or dish of water. The carmine may be seen to be expelled forcibly 
from the upper smaller opening, while, if placed near the lower opening, it will be drawn 
in. It becomes apparent that the water is continually drawn in through the lower 
(inhalent) opening and passed out through the upper (exhalent). In view of the func- 
tions of the gills and the mantle, described on page 174, it may be understood that this 
stream of water not only serves the purpose of respiration but also that, as it is strained 
through the minute pores in the surfaces of the gills, it must yield up the microscopic 
materials that serve as food for the mussel. The position of the mussel, directed against 
the flow of the river, not only insures a more effective resistance should the current of 
the river be excessively strong, but it perhaps gives the mussel greater advantage in 
collecting the food floating with the current. In lakes where no regular current pre- 
vails mussels may lie with their axes in any direction, but the oblique position in the 
bottom is virtually constant for those that are not in movement. 

The advantages of rivers over lakes for the growth of mussels may readily be in- 
ferred. The mussel can draw in and strain only the water that is close about it, and in 
the quiet water of a lake or pond new supplies of food are brought to its vicinity only 
by the comparatively slow forces that cause the intermingling of the waters of the lake. 
In the steady current of a river, on the other hand, the same water is never strained 
twice by the same mussel and, besides, the action of the current tends to stir up the 
small organisms and nutritive sediment which abound in the surface scum of the bottom. 
Observations by Clark indicate that mussels in lakes feed more largely upon plankton 
than those in rivers, the latter of which contain in their stomachs chiefly detritus or 

81 



82 BULLETIN OF THE BUREAU OF FISHERIES. 

finely divided nonliving organic materials. The rate of growth of mussels generally is 
much higher and the size attained is greater in rivers than in lakes. Other factors 
than currents, of course, enter into consideration, and these will be discussed in the 
appropriate places. 

A chief condition of rich growth of mussels is a plentiful food supply, and not all 
rivers are alike in this regard. There are relatively fertile and relatively sterile rivers 
and lakes, and the fertility of streams is likely to correspond in a rather general way 
with the fertility of the lands from which the drainage is derived. Whatever materials 
suitable for the construction of plant tissues are brought into the waters are likely ulti- 
mately to be converted in great part into plant or animal life, and no little part of the 
plant life that is formed is likely to be converted ultimately into animal life. 

A primary condition for the formation of thick shells of good quality is the presence 
in the water of suitable minerals, principally calcium, and all of the important mussel- 
bearing streams are those whose tributaries flow from regions of limestone or other cal- 
careous deposits. Consequently it is the Mississippi Basin which largely supports the 
pearl-button industry, though shells of commercial value are also found in the Great 
Lakes and Gulf of Mexico drainages, and some in the Red River of the North. The 
streams of the Atlantic and Pacific slopes are almost or entirely barren of valuable 
shells. 

Many factors, indeed, enter into the suitability of waters for mussels, and of the 
various species of mussels — more than 500 in the United States — each has its special 
requirements; some will thrive where others will not. Much remains to be learned 
concerning the relation of mussels to their environment, and the subject is particularly 
complex because of the great number of species involved; but it will be attempted to 
place the several phases of that subject in general review in a later section on habitat 
(p. 94). It is the purpose of this section to give such a general account of the habit of life 
and the conditions of existence as is necessary to establish the peculiar dependence of 
fresh-water mussels upon the immediate environment. 

LOCOMOTION. 

As regards their place of abode, fresh-water mussels are very largely creatures of 
circumstances. Since they are not frequently seen in motion it is probable that most 
of them spend their lives after the period of infancy very near to the place where they 
first settle down. Nevertheless they can and do move, and certain species, principally 
the more elongate forms, manifest a condition of restlessness at times. 

All mussels are sensitive to some stimuli; a splash of the water near them, a touch 
on the edges of the mantle especially at the siphons, or the passing of a shadow over 
them, will cause the siphons to be withdrawn and the shell to be tightly closed. There 
are evidences to indicate that when the disturbance is severe, as when the mussel is 
taken entirely out of the water, or is exposed to the sun by an unusually low stage of 
water, or is affected by extreme cold, the withdrawal of the mantle is so extreme as to 
break the living connection with the edge of the shell, and thus to cause, when growth 
is resumed, an interruption line or plane in the shell which is present ever afterwards. 
(See p. 132.) 

The reaction to evident stimuli consists merely in closing up; there are times, 
however, when a mussel is impelled to change its position. The inoverrjent may then be 



FRESH-WATER MUSSELS. 83 

in a vertical direction, the mussel going down deeper into the bottom; rarely does it go 
completely beneath the surface of the bottom; more frequently the mussel moves 
horizontally, leaving a distinct path behind it, which reveals the direction and dis- 
tance of travel. Locomotion is accomplished by thrusting the muscular foot forward 
into the bottom, expanding the outer end, and then contracting special muscles so as 
to draw the shell and body nearer to the end of the foot. Mussels that are most likely 
to travel in this way are the yellow sand-shell, black sand-shell, and slough sand-shell, 
species that are relatively long and narrow. Rotund forms, like most of the species of 
Quadrula, are less likely to migrate, but, of the Ouadrulas, perhaps the most vagrant 
form is the very elongate rabbit's-foot, Ouadrida cylindrica.'^ The pocketbook, Lamp- 
silis ventricosa, and the pink heel-splitter, Lampsilis alaia, are also fairly active. Juvenile 
mussels are more active than adults. (PI. V, fig. 3.) 

The causes of movements from one location to another are not known and the 
subject offers an interesting field of study. Change of pressure (depth), temperature, 
or more probably light may be the governing factor. Yellow sand-shells move up on 
the shoals or toward shallow water in times of flood, and return toward deeper water as 
the stage of water recedes. It is a matter of common report that after high-flood stages 
these mussels are sometimes found stranded in the swamps at some distance from the 
ordinary channel of a river, but the authenticity of such reports is not established. 
Headlee and Simonton (1904, p. 175) observed that fat muckets moved away from shore 
during periods of high-wave action. 

Isley (191 4) tagged and planted large numbers of mussels in comparatively shallow 
natural waters and after several months recovered a considerable percentage of them, 
finding very little evidence of migration. The Quadrulas placed in water over 3 feet 
deep remained approximately where planted ; those placed in water as shallow as i foot 
moved to deeper water, which was easily reached. The species of Lampsilis used in 
the experiments showed more acti\aty, but none were discovered which had moved more 
than a few yards. He concluded from his experiments and field obser\^ations that 
mussels, especially the Quadrulas (heavy-shelled mussels) and related species, were 
unable to help themselves if conditions became unfavorable, but that, on the other hand, 
their power to endure unfavorable conditions was remarkable. 

From observations in Lake Maxinkuckee, Ind., Evermann and Clark (1918, p. 256) 
say: 

The rau.ssels in shallow waters near the shore move into greater depths at the approach of cold 
weather in late autumn or early winter and bury themselves more deeply in the sand. This movement 
is rather irregular and was not observed every' year. It was strikingly manifest in the late autumn of 
1913, when at one of the piers off Long Point a large number of furrows was observed heading straight 
into deep water, with a mussel at the outer end of each. The return of the mussels to shore during 
spring and summer was not observed. [These were mostly Lampsilis luteola, the fat mucket.] 

It is evident from the available data that the locomotion of fresh-water mussels 
can play little part in their distribution. Distribution is, in fact, effected principally 
during the period of parasitism on fish, when it is governed by the migrations of the 
hosts. When dropping from the fish, the little mussels are naturally subject to the 
force of the current, and some that fall in unfavorable environments may be carried to 
a more suitable place, while others falling upon good ground may drift into a less favor- 

» Wilson and Clark (1914. pp. 35 and 59) have noted a particularly vagrant habit for Quadrula cylindrica. 



84 BULLETIN OF THE BUREAU OF FISHERIES. 

able situation. Distribution by currents presumably has little practical effect, except, 
perhaps, in the case of such a thin-shelled species as ^4 yiodcmla imbecillis. 

DENSITY OF POPULATION. 

Strange stories are heard of the density of mussels in beds. It has been said that 
the living mussels in certain beds were in a layer 2 feet deep. Such stories, persistent 
among clammers, are, of course, based upon faulty reasoning. A bed is gone over 
repeatedly with crowfoot bars, and with continuing success, but the fact is overlooked 
that the appliance takes mussels only at random. A layer of mussels is not moved at 
each drag. A particular bed in the Mississippi River, more than a quarter of a mile 
long and 100 yards wide, was insistently described as being uniformly 2 feet deep in 
mussels. Further inquiry elicited the information that the bed was virtually cleaned 
up in a season and that about a half dozen carloads of shells were obtained. A simple 
calculation showed that, had the bed been as described, at least 30 trains of 100 cars 
each would have been required to move the shells obtained. Other stories relate to 
such observations as the taking of mussels by suction dredges after excavating deep 
holes in the bottom, no consideration being given to the possibility of a mussel falling 
in with the caving sand from above. 

In planting operations and in experiments involving the retention of mussels for 
considerable periods, if normal health and growth are desired it is important to know 
how closely mussels m.ay be crowded. The following observations are therefore offered. 

The place of densest mussel growth observed by the senior author in the Grand 
River, Mich., in 1909, yielded 52 living mussels of 6 species from a space 6 feet long by 
3 feet wide, giving a density of about 3 mussels per square foot. Clark and Wilson 
(191 2, p. 20) found a most favorable place for observations of density in the Feeder 
Canal, near Fort Wayne, Ind., which had been recently drained. The bottom of the 
canal had been abundantly populated with mussels and from i square meter they took 
81 mussels of 8 species, or about 7^ per square foot. At the place of greatest observed 
density in the Clinch River, Tenn., J. F. Boepple took 66 mussels of 10 species from an 
area which he estimated to be 4 square feet; if his estimate was correct, the density 
was 16K per square foot. 

At all of these places mentioned, mussels occurred in such striking and unusual 
abundance as to suggest to experienced observers the desirability of making actual 
counts. It is fair to assume, then, that the natural occurrence of more than three or 
four mussels per square foot over any considerable area is unusual and that plantings 
of large mussels in greater density are warranted only where the conditions are shown 
to be particularly favorable. 

Very small juveniles may safely be planted more closely. Howard reared for a 
season 217 juveniles in a floating crate iS by 24 inches, but the rate of growth among 
them was very variable. (PI. V, fig. 3.) In other rearing experiments at Fairport (con- 
ducted by F. H. Reuling) 2,006 juvenile sand-shells were obtained from a trough 14 
feet long and i foot wide, a density of 143.3 per square foot. In another trough of the 
same size, 3,016 juvenile Lake Pepin muckets were reared, a density of 215.4 P^r square 
foot. It is not to be assumed, however, that the young mussels would have lived long 
and grown normally while crowded so closely as were these. 



FRESH-WATER MUSSELS. 85 

BREEDING. 

The internal phenomena connected with reproduction are presented in connection 
with the discussion of life history (Part 2, p. i38ff). Wehave to do here only with the very 
few external manifestations which have been obser\'-ed as related to the breeding 
activities. 

The eggs are fertilized by sperm emitted into the water by males and taken in 
with the inhalent current of the female. In a few species the females, when about to 
spawn, are marked by a striking development of lurid colors and elongate flaps on the 
margin of the mantle about the inhalent orifice. In addition to the bright colors there 
are peculiar spasmodic movements of this part of the mantle. This peculiarity has been 
observed in a good many pocketbooks, Lampsilis ventricosa, in a few fat muckets, 
Lampsilis luteola, in some L. radiata, L. orhkulata, L. higginsii, and L. ovata (grandma), 
and in nearly all of the L. multiradiala which have come under observ'ation. (See Clark 
and Wilson, 1912, p. 54; Wilson and Clark, 1912, pp. 13, 14; and Evermann and 
Clark, 1918, p. 284.) Ortmann (191 1, p. 319) has described such flaps in Lampsilis 
■ventricosa and L. multiradiala. He obser\^es that when the gravid females are undis- 
turbed the marsupia are pushed outward, so that they project out through the inhalent 
opening and even a little beyond the shell, as previously figured by Lea. The waving 
flaps lie alongside the marsupia, and he attributes to them a function in promoting a 
current of water over the marsupia. It seems more probable that these conspicuous 
flaps, which sometimes suggest the appearance of small fish, may serve as a lure to fish, 
bringing them into desirable proximity to spawners when the glochidia are ready for 
extrusion, thus rendering the fish liable to infection and so increasing the chance of 
survival of the glochidia. The following is quoted from Wilson and Clark (1912, pp. 
13. 14): 

The mussels were thickly scattered everywhere, with especially dense beds along the shore. The 
small fish were again noticed playing about in the immediate vicinity of the spawning mussels. 

L. veniricosus has a habit of moving its bright-yellow siphon fringes, which are much enlarged during 
spawning, back and forth in the water. This undulatory motion seems to attract the small darters 
and minnows, particularly Notropis blennius, which could be seen darting in toward the fringes 
repeatedly. It also probably assists in furnishing fresh water for the respiration of the young mussels. 

.\t intervals during the undulations small numbers of glochidia are discharged from the brood 
chambers of the mussel .and carried out of the excurrent aperture. These glochidia are of the hookless 
type, and must be taken into the mouth of the fish that is to carry them during their parasitic period. 
We can thus understand the advanfcige of attracting these fish and keeping them in the immediate 
vicinity during the discharge of the glochidia. 

WINTER HABITS. 

Very little is known of the habits of fresh-water mussels in winter. Observations 
of rate of growth indicate that growth practically ceases during the very cold months. 
(See Isely, 1914; and also p. 132.) Microscopic studies of sections of shell indicate that 
there are numerous slight interruptions and resumptions of growth, corresponding to 
each period of winter, and these are no doubt related to the fluctuations of temperature 
in fall and spring. 

According to clammers, mussels cease to "bite" with the approach of cold weather. 
The observations of Evermann and Clark on the movement of certain mussels from the 
very shallow waters near the shore of Eake Maxinkuckee in late fall have been previously 



86 BULLETIN OF THE BUREAU OF FISHERIES. 

quoted (p. 83). They do not generally migrate or bury themselves, however, but 
simply become benumbed so that they respond very slowly if at all to such stimuli as 
the touch of the clammer's hook. Evermann and Clark (19 18, pi 256) also observed that 
mussels are not altogether inactive in midwinter : 

Occasional mussels were observed moving about in midwinter, even in rather deep waters. During 
the winter of 1900-1901 , an example of Lampsilis luteola, in rather deep water in the vicinity of Winfield 's, 
wasobserved to have moved about 18 inches in a few days. Its track could distinctly be seen through 
the clear ice. 

FEEDING HABITS. 

It has been previously noted that a mussel in normal condition on the bottom 
keeps a stream of water continually passing in through one of two siphonal openings 
and out through the other. The food is derived from this current as it passes through 
the gills. The manner in which the food is collected and taken to the mouth has been 
well described by Allen (i9i4,p. 128 ft) from studies conducted at the Indiana Universitj 
Biological Station, Winona Lake, Ind. 

The filaments of the gills are covered with cilia which intercept the particles contained in the water 
and prevent their passing through the gills with the water. They become entangled in mucus, and 
tlirough the action of these cilia such particles are wafted toward the mouth in streams. If they are of 
a harmless nature or of food value, they are permitted to enter the alimentar)' tract. During the incuba- 
tion of the glochidia, the female gives up a greater or less part of one or both of the gills for marsupial 
purposes. At this period these parts are of little use for respiration or for the collection of food. 

Cilia similar to those of the gills line the entire branchial chamber, cover all organs which come into 
contact with the water, and also line the alimentary tract. They are, as is always true of cilia, in con- 
stant motion during life; they act independently of nervous control and in a single plane. Their con- 
certed action is in the fonn of waves — resembling in appearance the passing of a breeze over a field of 
grain, or the movement of a bank of oars. The direction which these waves or streams take varies in 
the several organs. But all of the streams taken together are coordinated to accomplish a certain common 
end. * * * 

The mouth of the Lamellibranch lies nearly as far as possible from the external openings, just 
behind the anterior adductormuscle. It is thus well protected from the entrance of harmful substances. 
It is flanked above and below by the thin narrow lips. The upper lip is continuous with the outer 
labial palp on each side, while the lower lip is prolonged into the inner right and left palps. Most of the 
ciliar\' currents of the contiguoiis faces of the palps and of the lips are directed forward to the mouth. 
The outer or noncontiguous faces of both palps and lips as vrell as the edge of tlie inner face of the lips 
bear cilia which arc directed backward and away from the mouth. Thus particles which find their way 
between the palps are carried to the mouth. As will soon be seen, very little undesirable matter ever 
reaches the mouth or palps, but even here Wallengren (1905) has pointed out how selection and rejection 
may be made. 

* * * The inner surface of the labial palps, except their outer margins, are made up of minute 

vertical ridges, or furrows. These constitute a quite complex mechanism for the sorting of material. 
» * -r, 

Upon the ridges as elsewhere occurs a ciliated epithelium. But the ciliar>' currents are disposed 
in a unicjue manner. Upon tlie anterior slope of each ridge they are directed backward while those 
on the posterior slope lead foi-ward. This seeming conflict is not such in fact, because only one set 
of cilia comes into action at a time. The position of the ridges determines which set shall function 
at a given moment. Thus tlie after slopes are ordinarily brought uppermost, the ciliary currents leading 
to the mouth are upon the surface, while the cilia which lead from the mouth lie somewhat underneath 
the ridges. So long as no adverse stimuli are received, particles which lie between the palps are thought 
to be passed on forward from one ridge to another, to the lips and mouth. 

In the event that distasteful matter reaches the palps a reflex erection of the ridges brings upper- 
most the cilia leading backward and such material is returned from summit to summit to the edge of 
the palps and discharged into the mantle chamber. * * * 



fresh-water mussels. §7 

The entire epithelium touching the brajichial chamber is abimdantly supplied with glands which 
secrete a mucous substance. The mucus envelops and binds togetlier in strands the material to be 
trans]X)rted by the cilia. This is particularly Uue of those particles which are of a very distasteful 
nature. * * * 

Observers have differed widely in their notions of the ability of the mussel to select its food. To 
me it is evident that there are, to summarize, four points where such choice is exercised: 

(i) The labial palps, at the upper margin. 

(2) The labial palps, on the furrowed surfaces. 

(3) The mouth. 

(4) The incurrent siphon. 

As to the last, it is surromided by a row of pointed, fleshy papillae, having a resemblance to plant 
structures. These have two senson,' functions — tactile and gustator>' ; for upon being disturbed mechan- 
ically they are withdrawn into the shell, while a continued teasing, or a strong chemical stimulus results 
in the closing of the shell . 

Allen conducted experiments the results of which indicated that a mussel siphons 
a liter of water (about i quart) in approximately 42 minutes. From other observations 
he was led to infer that mussels pass food through the digestive system somewhat auto- 
matically or regardless of appetite, but that the secretion of digestive juices and the 
utilization of the food ingested may be controlled according to the needs of the mussel. 

Allen gives a list of diatoms, desmids and other algte, and miscellaneous food items, 
but without quantitative data or appraisal of the relative values of the different sorts 
of food and without reference to the presence of plant detritus in the stomachs. Seem- 
ingly he supposed, as did many others before him, that mussels subsisted almost exclu- 
sively upon living organisms. Data bearing on this question are presented in the 
following section. 

FOOD OF MUSSELS. 

SIGNIFICANCE OF THE PROBLEM. 

The fact that the rate of growth of mussels seems so directly proportionate to the 
thickness of the shell (p. 129), or, speaking from a physiological point of view, to the 
mineral requirements of the mussel — for the shell is chiefly mineral — leads naturally 
to the supposition that the limiting factor of growth is not the organic food supply, 
but the mineral food supply. This is a rather startling inference, since we are accustomed 
to view animals in nature as engaged in a fierce competition for food, their numbers and 
the luxuriance of grovvth being proportioned to the abundance of food available; and 
the food we ordinarily think of is the organic (animal and vegetable) substance required 
rather than the mineral matter. Yet, if it could be assumed that the food requirements 
of a floater mussel are of the same nature as those of a pimple-back, then, since in the 
same body of water the floater with its shell of paperlike thickness may attain a length 
of 3^ inches in two seasons, while the pimple-back with thick shell may not in the same 
period attain a length of more than about an inch, the conclusion would seem probable 
that the thick-shelled species was restricted in growth, not for deficiency of organic 
food, but for lack of the materials necessary for the formation of shell. The assumption 
proposed, viz, that the food requirement of the different species is virtually identical, 
although plausible and substantiated by some evidence, can not be accepted as finally 
proved. 

It becomes of importance to determine what is the food of fresh-water mussels, 
whether the requirements of different species are the same, whether there is serious 
competition for organic food between commercial and noncommercial species, and 



88 BULLETIN OF THE BUREAU OF FISHERIES. 

whether there is a sufficient food supply in water in whicli it is desired to promote an 
abundant growth of mussels. 

Three bodies of evidence bearing upon some of these questions are presented in 
the following pages. One is a summary of the observations by H. Walton Clark, which 
have been published elsewhere in part; another is a table embodying the results of 
Shira's studies of the 60 juvenile mussels taken in Lake Pepin (Shira, unpublished 
manuscript); the third comprises previously unpublished obser\'ations made in 1916 
by Franz Schrader," formerly scientific assistant in the Bureau of Fisheries. The last 
will be given first since the studies were directed more particularly at the questions 
just presented. 

OBSERVATIONS OF FRANZ SCHRADER ON FOOD OF MUSSELS. 

SPECIES STUDIED. 

Four species that were thought to be fairly representative were selected for investi- 
gation: The river mucket, Lampsilts ligamentina, the Lake Pepin mucket, Lampsilis 
luteola, the blue-point, Ouadrula plicafa, and the spike, Unio gibbosus. The first named 
is a typical river mussel, and one of the most important of all from the button manu- 
facturer's point of view — considering the quantity and quality of the shells together. 
Lampsilis luteola, a shell of fine quality, is predominantly a mussel of standing bodies 
of water, and is found to comprise 31.5 per cent of the entire shell output of Lake Pepin. 
Qtiadrula plicaia, also a good button shell, is evidently equally at home in stagnant 
and in flowing water. It is a member of a genus in general slow, ponderous, and heavy- 
shelled. Finally, Unio gibbossus is a form of little commercial importance because 
of its colored shell but is extremely common in some localities. Thus in Lake Pepin 
13 per cent of the shells were found to be of this species, and it was thought that if 
competition for food played an important part in mussel ecology, the presence of this 
valueless form might be detrimental to the commercial species, especially when occurring 
in such numbers as in Lake Pepin. 

FOOD CONTENT OF WATERS. 

The first step taken was to make a careful examination of the water. For this pur- 
pose samples were taken from v>'ell-known mussel grounds. A water sampler operating 
by means of valves that are closed through releasing the catch by a string was used. 
The sample of water taken at from 2 to 4 inches from the bottom was treated with 
formalin and the contents allowed to settle in the usual way. 

The solid matter thus obtained may be roughly divided into three groups: (i) Min- 
eral matter; (2) organic remains predominantly from plants (detritus); (3) plankton, 
chiefly green algse and diatoms. The proportions of these were extremely variable, 
varying not only with the season but also with changes in the river level. Plankton 
varied from less than i to more than 20 per cent. The remaining material comprised 
chiefly detritus, for, except after thaws or rains, the mineral matter seldom e:<ceeded 
5 per cent of the total of solids. 

Regarding the plankton, it may be said that relatively few forms made up the 
greater bulk. Thus, among green alga there were Scenedesmus, Selenastrum, Pedi- 
astrum, Cosmarium, and Volvox, the latter especially in the spring. In August greater 

a Included with his consent. 



FRESH-WATER MUSSELS. 89 

or lesser fragments of various thread algee, such as Spirogyra, increased in numbers 
until they outranked all others in importance. The list of diatoms showed these forms 
as important: Coscinodiscus, Synedra, Asterionella, and Navicula. In far smaller 
quantities but generally present were Gyrosigma, Tabellaria, Gomphonema, Epithemia, 
and a few others that are negligible for practical purposes. 

In both quantitative and qualitative constitution no appreciable difference was 
noticed between samples from Lake Pepin and those from the Mississippi River at 
Homer, Minn. 

FOOD DISCRIMINATION UNDER NORMAL CONDITIONS. 

Having ascertained the materials available in the water as a possible source of 
food, the next step was to determine whether the different mussels showed a preference 
or dislike for any of these constituents. This necessitated an examination of the 
stomach and intestinal contents of naturall}' feeding mussels. In every case the con- 
stituents of the material obtained from the stomachs corresponded to those found in 
a free state in the water. This refers not onl}' to the kind of material found but also to 
the percentages, which were discovered always to correspond, at least roughly, to those 
obtaining in the water at that period. Lastly, no difference was observed — ^in stomach 
or intestinal contents — among any of the four fonns of mussels concerned. Thus, 
under normal conditions no discernible degree of discrimination is e\inced. 

UTILIZATION OF FOOD MATERIALS. 

The question of the utilization of these materials was best solved by an examina- 
tion of the feces. It was astonishing to note that only about one-half of the green algae 
and diatoms were attacked to any degree by the digestive processes. In fact the 
green algae, with their often delicate cell walls, on many occasions did not even lose 
color. It was the detritus that underwent the greatest changes. The vegetable origin 
of this material was easily discerned under the microscope before digestion had taken 
place, but in the feces, after digestion, the substance was found almost always to have 
undergone a radical change in appearance and in structure. It was evidently attacked 
by the digestive processes to a much greater degree than the plankton. 

These observations point to a comparatively unimportant role as played by algae 
and diatoms in the food of mussels. Not only are these forms present in very much 
smaller amounts than the dead-food materials but also they are not digested as well. 

EXPERIMENTS IN FEEDING VEGETABLE MATTER. 

Although under normal natural conditions no discrimination of food was observed, 
conditions might easily arise which would bring about a radical change in the constit- 
uents of this normal food supply. Feeding v/ith different materials was tried, therefore, 
to determine any preference that the mussels might have. 

The method used was to starve the mussels for four to five days and then feed 
them with the material under investigation. In this way the intestine was first cleared 
and the state of digestion of the fed material determined without any disturbing con- 
tamination from substances previously present in the intestine. As starved mussels 
may lose their sense of discrimination to a certain degree, an equal number of control 
mussels feeding and living in a tank with a flow of river water were always experimented 
on at the same time. The food was administered from a long pipette into the intak- 
ing siphon. It is unnecessary to go into the details here, but it may be mentioned that 



9^ BULLETIN OF THE BUREAU OF FISHERIES. 

the utmost care had to be employed so as not to feed particles of food exceeding a 
certain size. A neglect of this caution invariably caused violent expulsion of the 
whole dose of food, no matter what it was. 

ThrBad Alg^. — These were accepted by all four species, but only in limited 
quantities b)' the mucket, Lanipsilis Ugamentina. An examination of feces confinned 
the previous observations that green algae are only very incompletely digested. 

PalmellalES. — These soft, slimy, green algae gave no other result save that they 
seemed somewhat better digested by the blue-point mussel. 

Detritus. — This was artificially prepared by immersing the leaves and soft stalks 
of plants that are generally found near the water in some water for a few days imtil 
nitrogenization had set in. They were then macerated with mortar and pestle and the 
resulting pulp strained through bolting cloth. All mussels took this artificial detritus 
readily, and the feces showed the characteristic features of digested detritus. 

Fresh Vegetable Material. — This was not so readily taken. 

Vegetable Fat. — Olive oil in the form of an emulsion was accepted by the Lake 
Pepin mucket and the river mucket. It was evidently thoroughly absorbed, as no 
traces could be found after digestion. The blue-point and the spike were not tried with 
this material. 

experiments in feeding animal matter." 

Fish Meat (heart of the wall-eyed pike, Stizostedion vitreum). — Two out of three 
examples of the Lake Pepin mucket accepted this material readily, the third less so. 
The control mussels vacillated, occasionally taking very small quantities. The other 
three species evinced strong repulsion, expelling any of the substance taken in in from 
I to 15 seconds. Abnonnal reddish feces. 

Tails of Tadpoles (macerated). — These were refused or quickly expelled by 
the river mucket and the Lake Pepin mucket. The other mussels were not tried with 
this material. 

Blood of Pickerel.— This was refused by all species, even when given in a state 
of high dilution. 

Animal Fat. — This was an emulsion of fat obtained from the sheepshead fish, 
Aplodinotus grunniens. Extremely small doses at long intervals were taken and evi- 
dently digested. 

In all these cases the food matenal was generally readily taken (from i to 1 5 seconds) 
into the siphon. After a varying period of time (a few seconds), the length of time 
necessary for the substance fed to affect the taste organs, disagreeable food was always 
expelled again. 

The experiments do not point to any undoubted conclusion regarding animal food, 
except that they seem to establish the fact that vegetable food is preferred to the animal 
substances employed. Probably, under normal conditions, small quantities of the 
latter are taken in with other substances, but it is hardly believed that it ever plays 
a large role. 

general observations. 

Throughout the experiments it was noticed that the Lake Pepin mucket, Lamp- 
silis luieola, was not so exact in its requirements as the river mucket, Lampsilis Uga- 
mentina. The latter was indeed the most delicate feeder of the four species, and the 

a Allen C1914, p. 138) fed mussels upon living Paramecia with apparent success. 



PRESH-WATER MUSSELS. 91 

greatest care had to be taken in handling it, while just the opposite was true of its near 
relative, the Lake Pepin mucket, which fed readily on most of the experimental material 
and was not so fastidious regarding the physical state of the food; that is, the size of 
food particles and the amount given at one time. This may explain to a certain degree 
the success attending the culture of the latter species in ponds, but the question then 
arises why the river mucket is not crowded out everywhere by this mussel, since one 
species, judging from the shell structure, is as well adapted to live in moving water as 
the other. As was observed above, however, Lant.l>siHs luteola is typically an inhab- 
itant of water with little or no current, while Lampsilis ligamentina is a true river mussel. 
The available data of shell structure and feeding habit evidently offer no explanation. 

The blue-point and the spike take a midway position as regards their feeding 
habits, although the former is perhaps less exacting than the latter. 

Detritus undoubtedly forms the main bulk of the food of fresh-water mussels. 
Dissolved substances may also play a part (Churchill, 191 5 and 191 6), but their role is 
probably a comparatively unimportant one when compared with the solid food matter. 
This must be especially true of streams with relatively pure water, in which mussels 
have been found to thrive just as well or better than those carrying large quantities of 
dissolved matter. 

In view of the universal presence of plants in or near waters productive of mussels 
there is little likelihood of a shortage of food, for detritus will always be forthcoming. 
There can be only a very little competition among mussels as far as food is concerned, 
and the noncommercial species are not objectionable from this standpoint. 

OBSERVATIONS OF H. WALTON CLARK ON FOOD OF MUSSELS." 

In general it may be said that the food of fresh- water mussels, as indicated by their 
stomach contents, includes about everything obtainable and not positively harmful, 
organic or inorganic substances, living or dead matter, if not too large or too active for 
the mussels to take in. As the mussel has no means of mastication it can not use long 
objects such as filaments of algse and the like. 

In the course of general biological investigations and of mussel surveys opportunity 
was had to study the stomach contents of mussels from widely separated areas and 
under widely different conditions. One of the striking features of the case is that the 
size and apparent health of mussels bear no direct relation to the apparent nutritive- 
ness of the material in the stomach. Thickness of shell is partly a matter of heredity; 
thick-shelled species of Lampsilis are found in fairly good currents where nutritious food 
material is scarce; thin-shelled Anodontas are usually found in quiet places where the 
food supply is rich. Moreover, generally speaking, Lampsilis of any species in a quiet 
lake where food in the form of plankton is abundant, are thinner shelled and smaller 
than those of rivers. 

Although, generally speaking, thickness of shell seems to be almost always in 
inverse ratio to richness of food, that relation itself may be partly accidental. In mus- 
sels the secretion of shell is in relation to current or to mineral content of the water. 

The stomach contents of some large heavy pocketbooks, Lampsilis ventricosa, 
from the mussel beds in Yellow River, Ind., where this species reaches maximum size, 

a For additional data see Clark and Wilson (igi2) and Evennann and Clark (1918). 

9745°— 21— 2 



92 BULLETIN OF THE BUREAU OF FISHERIES. 

consisted chiefly of the yellow mud of the river bottom, with organisms of any sort few 
and far between. In general, mud" is an abundant element in the stomachs of all mus- 
sels; so much so that the color and general appearance of the mass of the stomach con- 
tents of all river mussels examined was that of the bottom soil. In ponds full of dif- 
fused plankton algse the plants may be present in sufficient quantities to at least fleck 
the "ground color" with a pronounced green or blue green. Studies of the stomach 
contents of the mussels in the reservoir of the Feeder Canal at Fort Wayne, in 1908, 
revealed the presence of many flagellates, such as Trachelomonas and Phacus, together 
with such minute plants as Scenedesmus, Pediastrum, Botryococcus, such diatoms as 
Gomphonema, Na\'icula, and the like, a few desmids (Cosmarium), fragments of Cera- 
tium hirundinella, casts of the rotifer Anuresa cochlearis, and small fragments of 
confervoid algae. In the main current of the vSt. Joseph, St. Mary, and Maumee 
Rivers there was much mud with about the same organisms scattered sparsely through 
it. A mucket, Lampsilis ligamentina, taken in the Auglaize River, contained what 
appeared to be bacteria. Mussels in Lake Amelia, near ,St. Paul, Minn., contained an 
abundance of that peculiar organism Diiiobryon serlularia. The mussels of Lost Lake 
and Lake Maxinkuckee, Ind., contained enough plankton organisms of all the minuter 
sorts to give the stomach contents a greenish cast or to mottle it considerably with greenish 
flecks. Not to enter into too great detail, the)' contained such organisms as Microcystis 
(BTuginosa, Pediastrum boryanum, and P. duplex, Coelastrum microporum,, Botryococcus 
braunii, Scenedesmus, Melosira crenulata, Coconenia cyrnbifornie., Navicula, Epithemia 
argiis, Fragilaria, Cocconeis pedicidus, and Lyngbya tzstu-arii. Melosira and Spirulina 
represented the longest filaments taken. Anurcea cochlearis was common but represented 
only by lorica, and Chydorus was the largest and most active organism taken. 

Observations believed to be of both interest and importance were made in the Mis- 
sissippi in the late summer and autumn of 1919. The river had remained high and 
swift until about the beginning of September, when it fell rapidly. With its fall the 
great body of marginal water lost the velocit}' of its flow, and great areas behind wing 
dams, lagoons, and mouths of sloughs became extensive areas of calm. In these a rich 
and varied plankton, consisting chiefly of holophytic sorts (Euglena, Paudoriiia, rotifers, 
Platydorina, and a bottom benthos of diatoms), rapidly developed in considerable quan- 
tities. The stomachs of the mussels in the bottom of these areas of calm contained 
numerous organisms of the plankton and benthos such as A nurcea cochlearis, Pandorina, 
Mycrocystis, Scenedesmus, Phacus, and various diatoms; the stomach contents bore 
general resemblance to those of the mussels of the Feeder Canal reservoir. 

Opportunity was taken to examine the stomach contents of some young mussels 
which were obtained at the same time. In a slough sand-shell, Lampsilis jallaciosa, 
1 9. 1 mm. long, all that could be recognized was one colony of Clathrocystis. Another, 
18.9 mm. long, contained chiefly brown, gritty mud in which were several Scetiedesmus 
caudatiis, Pliacus plcuronectcs, i Coscinodiscus, a few x&xy minute Melosiras, and some 
rough spherical cysts. A third example, 19.6 mm. long, contained much brown flocculent 
organic mud, a large colony of Microcystis, i Scenedesmus caudatus, the diatom Cyclotella 
compta, and many of the green rough cysts. 

The stomachs of some very small Lampsilis anodonloides and L. luteola, reared in 
troughs at Fairport and apparently thriving, contained only a fine brown flocculent 

1 The mud is probably mixed with much decomposing organic matter. 



PRESH-WATER MUSSELS. 



93 



mud, with rarely an occasional diatom. A young L. anodontoides from Smiths Creek bar 
in the Mississippi contained fragments of diatom shells indicating that it had been feeding 
on them to an unusual extent. Although Pleurosigma covered the nmd of that region, 
forming an almost unbroken brown scum, it is noteworthy that it was only rarely found in 
the stomachs of the young mussels, it being apparently too large to enter the mouth. 

As regards the entire subject of mussel food and feeding there are some general 
obsei-vations it may be pertinent to make at this point. 

At one time it was thought that extremely dense beds of mussels in the bottom of 
lakes might act as reducers of an excessive accumulation of plankton. They might 
indeed take care of many sunken and decaying plankton organisms, but under favor- 
able conditions plankton can develop more rapidly than anything can eat it. 

The finding of what appears to be bacteria in the stoniachs of mussels of the Auglaize 
River and the observation made in tanks at the Biological Station at Fairport — that 
turbid water in which there were mussels cleared up rapidly, the mussels collecting the 
silt and other materials in suspension — raise the question as to whether mussel beds are 
not or can not be of use in the purification and sanitation of rivers. If oysters grown 
m polluted waters may harbor typhoid bacilli and so communicate the disease to those 
who eat them, there seems to be no good reason why mussels, which are not eaten, may 
not sen'e to arrest and devour those as well as other pathogenic organisms. 

Since mussels are very inactive animals, the rate of metabolism may be expected 
to be low and the food requirements correspondingly small. The problem of obtaining 
nourishment for mussels is then one of the least of our troubles. Doubtless younger, 
more active mussels require a richer diet, and the first problem of mussel propagation, 
that of finding a suitable host, is fundamentally one of finding suitable nutrition for a 
creature remarkable for its fastidiousness in this regard. It ma}- be that a critical 
problem is the finding of suitable nourishment for the first month or so of free life, but 
beyond this the only problem, so far as food supply is concerned, appears to be the 
avoidance of actually poisonous or harmful substances. 

OBSERVATIONS OF A. F. SHIRA ON FOOD OF JUVENILE MUSSELS. 

The following table (i) embodies a record of the stomach contents of 60 juvenile 
mussels, distributed among 6 species, taken in Lake Pepin during 1914. The material 
v/as studied with the use of a rafter counting cell, but since only a very small quantity 
of food could be obtained from each mussel the calculation of percentages can be only 
approximate. 

Table i. — Food of Six Species op Juvenilb Mussels Taken in Lake Pepin, September, October, 

AND November, 1914. 



Juveniles examined. 




Length 


of juvenile 

meters). 


s (milli- 




Percentage of food . 




Spedes. 


Number. 


Mini- 
mum. 


Maxi- 
mum. 


Average. 


Organic 
remains 
(princi- 
pally 
vege- 
table 
matter). 


In- 
organic 
remains 

Csilt. 

etc.). 


Unicel- 
lular 
green 
algse. 


Diatomi 


T.iiTnpQilis liitppla 


13 
10 

8 
10 

8 
12 


5-0 
4.8 
8.2 
6.6 
6.4 
6.2 


IS- 4 
13-4 
21.5 
19. 6 
II. 
19- 5 


9-5 
8-8 
13- 7 
'3- 5 

r-4 

12. 7 


S9 
90 
96 
05 
96 
91 


6 
5 

I 

X 

Trace. 
4 


3 
3 

2 

3 
3 
3 








Lampsilis alata 








Quadrula plicata 













94 BULLETIN OF THE BUREAU OF FISHERIES. 

HABITAT. 

The pearly mussels, as inhabitants of fresh water, are found in diverse habitats, in 
lakes and in rivers, in shallow and in deeper waters, in cold and in warm waters, in mud, 
in sand, and among rocks. Yet they do not occur in all lakes and rivers, nor in all parts 
of the lakes and rivers in which they do live; and the several species of mussels, when 
living togetlier, are not always found in the same relative abundance. It may, therefore, 
be supposed that fresh-water mussels, like other animals, are adapted rather definitely 
to particular conditions of environment; that some find congenial environment in still 
or sluggish water, while others thrive best in strong currents ; that a mud bottom supports 
certain species, while a firmer soil is required by others. 

Adult mussels in some cases thrive, or continue to live at least, in environments 
where the young would perish, for delicately balanced conditions are required by very 
young mussels of many species, and only where these conditions exist can a mussel bed 
originate or perpetuate itself. On the degree of stability of the conditions favorable to 
the growth of the young the permanency of the bed must depend, since, when replenish- 
ment fails, the bed can continue only as long as the life of the adult mussels it contains. 
As any mussel has rather limited powers of independent locomotion, the place where it 
lives (or prematurely dies) is probably, as a general rule, near where it falls when it 
drops from its fish host; yet the early juvenile can be carried by the current, and doubtless 
this means of transportation may sometimes aid the young mussel in finding a suitable 
habitat. An adult niggerhead mussel lived in apparently healthy condition in a balanced 
aquarium at the Fairport station for nearly nine months; yet in nature this species is 
found only in strong currents, the favored environment of its fish host, the river herring. 

The relationship of fresh-water mussels to the environment may be treated with 
reference to body of water, bottom, depth, light, current, water content, vegetation, 
and animal associates. 

BODY OF WATER. 

The various geographic types of fresh water in which mussels occur are rivers, lakes, 
ponds, sloughs, swamps, marshes, and canals. In so far as distinctive conditions char- 
acterize these various types of waters, each may have its characteristic mussel fauna. 
It may be said in general, that wherever conditions suitable for a particular species of 
animal prevail, that species will be found, except as it may have been naturally excluded 
through features of geologic history or other factors governing the distribution of animals; 
in the case of fresh-water mussels, however, emphasis must be placed upon a qualifica- 
tion of this general statement. Though all conditions in a body of water may be other- 
wise suitable, mussels can not naturally occur where conditions do not permit the entry 
and survival of the species of fish which serve as hosts. 

STREAMS. 

Mussels have undoubtedly reached their greatest development, as to numbers, both 
of species and of individuals, in flowing water. From the commercial standpoint, also, 
the quality of shells from streams is almost invariably superior. In general, where other 
conditions are favorable to mussels, larger bodies of flowing water are more productive 
than the smaller. Brooks do not usually contain mussels. Morphologically, mussels 
adapted to life in strong currents are differentiated from those adapted to still water by 



Bull. U. S. B. F., 1919-20. 



Plate VI. 




Fig. I.— Upper waters of Grand River. Habitat of mussels in sliallow swift water. 











^ -JL>^ 


) JS^^^^^^ 


in 




1^ 




i 






'^^^^^^1 




f 




r 






^^BBB 




w 






.^0^ 




Wr ' ■ '^4^^H 




*"-• 






--i« 


w '-^^^H 


^H|E^~ -^^^^^^^^^^^B 



Fig. 2.— Upper waters of Grand River. Habitat of mussels in sluggish water. 



Plate VII. 




Fig. I. — Black Kult, Ark , a very productive mussel stream. 




Fig. 2. — Red River, near Campti, L,a., a turbid stream with caviny banks and shifting bottom, quite unfavorable 

for mussels. 



Bull. U. S. B. F., 1919-20. 



Pi.ATK MIL 




Tig. 



-Lake Pepin, an expansion of the Mississippi River between Wisconsin and ^Minnesota, 
a favorable habitat for fresh-water mussels. 




-North Fork of Kentucky River, near Jackson. K^■.. with sand bottom and conditions unfa\-ora 

( r);illi,'l;ido I 



)k' fur mussels, 




-Tea Table shoals, another portion of the Kentucky- River; the shores indicate stabjlii> 
moderately deep, and the environment is favorable for mussels. 



Bull. U. S. B. F., 1919-20. 



Plate IX. 




Fig. I. — An undredged portion of the Kankakee River where valuable mussels lluurisli. 




^Mi^^ 



Fig. 2. — A dredged portion of the Kankakee River rendered (temporarily, at leastj unfit for fresh-water 

mussels. 



Bull. U. S. B. F., 1919-20. 



Plate X. 




Fig. I.— Lower porlion of Grand River. Mich., where mussels thrive under natural conditions. 




Fig. 2. — Lower portion of Grand River where conditions have been rendered unsuitable for mussels by 
canalization in interest of navigation. 



BuLU U. S. B. F., 1919-20. 



Plate XL 





Fig. I. — Auiilaize River near Defiance. Ohio, showing 
islets and pools coutaiumy dense beds ol mussels. 



l'"ii.. _■ — M.iiinuc River, ru-fiaiice Olii... The river 
heii a broad valley with limestone bottom, broken 
into numerous pools and channels with little islands 
— an excellent growth of fresh-water mussels. 





Fig. 3.— The drunim m1 li:, 
Wayne. Ind.. r'.'\Lalni a i 
tion of fresh-water mussels 



1'\t'iKt Canal near Fort 
inarkab!>' dense popula- 



FiG. 4. — Parts of the Miami and Erie Canal afford 
excellent environments for mussels. 




Fic 



5. — Construction of wing dams in the upper Mississippi River often renders conditions unfavorable for mussels that 
previously throve in such sections of the river. 



FRESH-WATER MUSSELS. 



95 



the stronger development of the hinge teeth which aid in keeping the two valves of the 
shell in perfect apposition. 

Since a river presents from source to mouth conditions of varying suitability for any 
form of animal life, there will usually be found in some measure a longitudinal succession 
of mussels. Shelford (1913, p. 122) gives a table showing the longitudinal sequence of 
eight species of mussels in the Calumet Deep River. 

If one goes down a river from its headwaters, making collections of mussels at 
various points, many species may be found at each place, but some species first encoun- 
tered may disappear before the upper waters are passed. Others appear here or there 
and perhaps disappear as one proceeds still farther down. The mussel fauna of the 
different sections of the stream are characteristic, although one or more species may be 
so adaptable as to live throughout the entire course of the stream. 

This longitudinal succession of species is well illustrated by Table 2, which shows 
the distribution of mussels in the Grand River, Mich. 

Table 2. — Longitudinal Distribution op Mussels in Grand River. Mich.<i 



Scientific name. 



1. Quadrula coccinea 

2. Strophitus edentulus 

3. Anodonta grandis . , 

4. Lampsilis ventricosa 

5. Quadrula rubiglnosa 

6. Anodontoides ferussacianus. 

7. Lampsilis iris 

8. SymphiTiota compressa 

g. Lampsilis luteola 

10. Alasmidonta calceola 

ir. Unio gibhosus 

12. Symphynota costata 

Lampsilis elHpsifonnis 

14. Quadrula undulata 

15. Lampsilis lipamentina 

16. Alasmidonta marginata 

17. Quadrula tuherculata 

18. Lampsilis recta 

19. Lampsilis alata 

20. Quadrula pustulosa 

21. Lampsilis gracilis 

22. Obliquaria reflcxa 

33. Obovaria ellipsis 

24. Plagiola elegans 

25. Quadrula lachrymosa 

26. Symphynota complanata 



13 



Total. 



Common name. 



" Flat niggerhead ' 

Squaw-foot 

Floater 

Pocketbook 

Flat ni ggerhead . . . 

Small floater 

Rainbow-shell. . . . 



Fat mucket . . 
Slipper-shell. 

Spike 

Fluted shell, . 



Three-ridge 

Muckct 

Elk-toe 

Flat purple pimple-back 

Black sand-shell 

Hatchet-back, pink heel-splitter. 

White wart y-back 

Paper-shell 

Three-homed warty-back 

Hickory-nut 

Deer-toe 

Maple-leaf 

White heel-splitter 



Observations made at and below- 



— 01 4^ 

O 



14 



14 






X I X 
X I X 

X 

X X 

. . . . X 



16 



o Observations by R. E. Coker in 1909. 

Stjmmarv or Table. 

Total species observed. 26 

Other species occurriug above Portland 6 

Species occurring throughout river 10 

Species found only at or below Portland 10 

Species not found below Grand Rapids 4 

Species found only below Grand Rapids 6 



It will be observed at once that a far greater number of species is found in the lower 
part of the stream. Thus, while only to of the 26 species obser\'ed in the river were 
found near the headwater lakes, 22 species were met in the section of the river between 



96 BULLETIN OF THE. BUREAU OF FISHERIES. 

Grand Rapids and the mouth at Grand Haven. It might be thought that this was due 
to the fact that there would be fewer obstacles to the passage of mussels downstream 
than to their distribution in an upstream direction. It seems sufficient, however, to 
assume that the unequal distribution is due rather to the greater variety of conditions of 
depth and of fish associates presented in the lower portion of the river. Shallow water 
only is found in the upper river, except as artificial pools have been formed in recent 
years by the construction of dams, while in the lower river deep water prevails in its 
channel and all lesser depths are found between the channel and the shores. The very 
breadth of the lower part of the river affords also a greater area for fish and mussels. 

A difference of up-river and down-river habitat is presented by the distribution of two 
closely related species, the three-ridge, Qiiadrula undulata, and the blue-point, Quadrida 
plicata; the former, a more compressed and rougher form, is found in the more rapid 
waters of upstream habitats, while the latter, being thicker and less ridged, occurs in the 
deeper waters of the lower parts of a river system." (See Clark and Wilson, 191 2, and 
Wilson and Clark, 191 2.) 

In some rivers mussels are almost entirely lacking for long distances, as in the main 
course of the Missouri River for hundreds of miles above its mouth, where the absence 
of mussels is apparently due to the rapidly shifting bottom of sand. The Red River, 
with its heavy load of silt and its habit of suddenly cutting into its banks and changing 
its course, is manifestly unsuited for mussels, and examination of its bottom in many 
places by Isely (1914) and Howard revealed extremely few mussels (PI. VII, fig. 2). There 
is also a virtual absence of mussels in the Mississippi River, except close alongshore, below 
the mouth of the Missouri River. Examination of the Musselshell Ri\'er in Montana by 
J. B. Southall in 1919 revealed the presence in numbers of only a single species of mussel, 
and this a species {Lampsilis luteola) characteristic of lakes, which lived in the portions 
of the river deep enough to remain as isolated pools during the periods of dry weather. 
In the east fork of the Chicago River, Baker (1910) found only 3 species, and these were 
mussels characteristic of pond habitats, which were able to survive the dry seasons in 
the small ponds left isolated in the deeper parts of the river channel. 

The James River, in North and South Dakota, though having very few fish, was found 
to possess a comparatively varied and abundant mussel fauna in the still waters between 
shallow riffles; but there was evidence that the mussels were derived from fish infected 
in other waters, that ascended the stream iu times of flood (Coker and Southall, 1915). 

The suitability of any section of a stream for the growth of mussels arises from a 
diversity of causes, including the nature of the rock or soil through which the stream is 
flowing, the character of the drainage waters entering the river at or above the section, 
the gradient of the stream bed with its effect upon depth and currents, and the species 
of fish which frequent the region. 

Barriers in the course of a stream such as natural falls, or artificial dams, if impassa- 
ble to fish, may have an effect upon the distribution of mussels. Wilson and Danglade 
(1914) found no mussels of the genus Quadrula above the F'alls of St. Anthony in the 
Mississippi River, although several species of this genus are very common in the river 

n Ortmann (1920) has definitely shown, for certain species, that: "(i) The more obese (swollen) fonn is found farther down 
in the large rivers, and passes gradually, in the upstream direction, into a less obese (compressed) form in the headwaters; (2) 
with the decrease in obesity often an increase iu size (length) is correlated; (3) a few shells which have, in the larger rivers, a pe- 
culiar sculpture of large tubercles, lose these tubercles in the headwaters." He ascertains also that these laws do not apply to all 
species. 



FRESH-WATER MUSSELS. 



9V 



below that barrier. Wilson and Clark (1914) found only 4 species of mussels in the 
Cumberland River above the Cumberland Falls (one of these probably planted), while 19 
species were taken in the pool immediately below the falls, but in this case the conditions 
prevailing in the river above the falls appeared distinctly unfavorable for fresh-water 
mussels. An impassable dam formed after mussels were generally distributed throughout 
a stream would have little significance with reference to the distribution of mussels tlie 
hosts of which subsequently thrived both above and below the dam. The effect, how- 
ever, of a dam in changing a region of rapids into a pool, might cause the mussel fauna of 
swift waters to give place to a fauna of slack-water habitat. 

Studies of rivers in cross section indicate that there may be quite definite distribu- 
tion of life with reference to the banks. Shelford (191 3) has discussed a horizontal 
arrangement of animals that is best illustrated in the cross sections of curves where there 
is a horizontal gradation in rate of current and in size of material in the bed of the stream. 
In the strong current only the coarsest materials are dropped, while the finest silt is 
deposited where the flow is most retarded. The depth, of water is doubtless one factor 
governing the hoi'izontal distribution of mussels, but the nature of the bottom material 
is of first importance. Howard (Survey of Andalusia Chute, Mississippi River, report in 
preparation) foimd in a branch, of the Mississippi, following a comparatively straight 
course (not on rapids) and averaging 1,200 feet in width, that mussels were uniformly 
restricted to a border 200 feet from the shore line. (See table below.) vSome mussels 
vv'ere found almost anywhere along this border, but occurring in beds at points where the 
channel touched the shore and where bottom conditions were favorable; depth seemed 
to be a minor factor as affecting the distribution. 

The following table (3) indicates the results of a sample series of unit hauls taken at 
stated distances from the water's edge and so represents the distribution in a cross 
section of the river. It is not typical because of the narrowness of the bed on the left 
bank, but it illustrates in a general way the distribution found throughout the survey. 

Table 3. — Distribution op Mussels in Andalusia Chute, Mississippi River. 







From right bank. 




Middle 

of 
river. 


From left bank. 


Distance, in feet, from water's edge. . 


S5 

13 


8s 100 
3 


200 



300 


400 


300 




200 



no 



60 
12 






33 









Where there arc rapids with bowlder or cobblestone bottom across a river of this 
size, it is known that mussels are not limited to such a border but are found at all points 
across the stream. 

LAKES. 

The mussels from some lakes are large and heavy-shelled, while in others thev are 
small, thin-shelled, and stunted. These extremes represent the varied conditions 
which lakes present in respect to mussel life. 

Lakes that have a free circulation of water seem to be favorable; such are those 
that are interposed in the course of a river. A favorable feature in such cases, no doubt, 
is the direct and free connection witli streams that are well supplied with mussels. 
Examples are Lakes Pepin and Pokegama, Minn., and the former is noteworthy for the 
abundance of mussels produced. Though at first sight Lake Pepin might be considered 



98 BULLETIN OF THE BUREAU OF FISHERIES. 

no more than an expansion of the Mississippi River, further observation shows it to be 
a true lake in many of its characters, as in clearness of the water, depth, growth of vege- 
tation, and virtual absence of current. In both of the lakes mentioned, a characteristic 
lacustrine species, the fat mucket, or Lake Pepin mucket, Lampsilis hileola, which is 
thin-shelled and worthless in ordinary inclosed lakes, attains so fine a commercial quality 
of shell as to appear almost as a distinct variety. Caddo Lake, La., which is interpolated 
in the course of a stream, possesses a rich mussel fauna and has been the scene of active 
pearl fishery (Shira, 1913). The small Rice Lake near La Crosse, Wis., which is, in 
effect, an expansion of a thoroughfare connecting the Black River, near its mouth, with 
the Mississippi River, also supports a varied and luxuriant mussel fauna. Where lakes 
are freely connected with rivers, as are those first mentioned, or as are others with short 
open outlets to the rivers, the lakes and rivers have many species of mussels in common. 

In Lake Pepin, Shira (report in manuscript) found that the distribution of the mus- 
sels is confined wholly to the shore line and the flats within a maximum depth of 25 feet; 
no mussels at all were taken in the deep central part of the lake. In certain places the 
mussels were quite densely distributed, forming very well-defined beds, but as these 
beds were generally connected by areas of lesser population, a more or less continuous 
mussel bed was found to occur on each side of the lake. The largest and most extensive 
beds were located on a gravel bottom, or a mixture of gravel and sand. Several good 
tliough less extensive beds occurred on bottoms containing a considerable percentage 
of mud. 

The upper end of the lake evidently serves as a settling basin for the silt poured in 
from the river proper, and for a distance of about 2 miles below the entrance of the river 
the lake is comparatively shallow with a soft oozy bottom. In this section of the lake 
very few mussels are found. 

Shira records 32 species of mussels (report in manuscript). Ten of the most abund- 
ant species with the percentage of occurrence are given as follows: 

Per cent. 

Fat mucket, Lampsilis luieola 31-5 

Spike, Unio gibbosus 13. o 

Blue-point, Quadrula plicaia 12. 7 

Pig-toe, Quadrula undala 10. o 

Pink heel-splitter, Lampsilis alata 8. 3 

Pocketbook, Lampsilis ventricosa 5. 6 

Slop-bucket, Anodonia corpulenta 5. 5 

Squaw-foot, Slrophitus cdcniulus 4-3 

White heel-splitter, Symphynota complanala 2.3 

Black sand-shell, Lampsilis recta 1.4 

In small lakes of considerable depth and without circulation, except as effected by 
winds and changes of temperature, animal life generally is absent or greatly restricted 
in the deeper portions, and mussels, when present, are confined to zones near the shores 
(Headlee and Simonton, 1 904) . Muttkowski ( 1 9 1 8) in Lake Mendota found the optimum 
conditions for mussels at depths of 6 to 9 feet on sand bottom, but there was not an 
extensive mussel population in the lake as a whole. 

The restriction of mussels to the border zones is indeed generally characteristic 
of the lakes of the Middle Western States, and even in this environment where the cir- 
culation effects of wave action may be felt, the mussels are stunted in growth. In their 
report on the mussel faiina of Lake Maxinkuckee, Evermann and Clark (1918, p. 251), 



FRESH-WATER MUSSELS. 



99 



summarize as follows the results of observations of mussels in lakes of Indiana and else- 
where : 

Generally speaking, lakes and ponds are not so well suited to tlie growth and development of mussels 
as rivers are; the species of lake or pond mussels are comparatively few and the individuals usually 
somewhat dwarfed. Of about 84 species of mussels reported for the State of Indiana, only about 24 are 
found in lakes, and not all of these in any one lake, several of them bat rarely in any. Of the 24 species 
occasionally found in Indiana lakes, but 5 are reported only in lakes, and only 3 or 4 of the species com- 
mon to both lakes and rivers seem to prefer lakes. 

Characteristic species of mussels of inclosed lakes of upper Central States are named 
in the following table (4), and it may be remarked that the fat mucket and the floater 
are easily predominant over all others. 

Tabue 4. ^Characteristic Mussels op Lakes of Upper Central States. 



Species. 



Fat mucket, Lampsilis luteola 

Floater, Anodonta trrandis 

Pocketbook, Lampsilis veutricosa 

Squaw-foot. Strophitus edentulus 

Small floater. Anodontoides ferussacianus. . 

Quadrula rubicinosa 

Spike, Unio Eibbosus 

Lampsilis subrostrata 

Rainbow-shell. Lampsilis iris 

Slop-bucket, Anodouta corpulenta 

Paper shell, Anodonta imbeciUis 

Paper-shell, Anodonta pepiniana 



Michigan." 



Minnesota.'' 



Indiana c 



" Cokcr. R. E. (unpublished notes). 

f* Wilson and Danclade (1914). 

c Clark and Wilson C1912); Wilson and Clark C1912); and Evermann and Clark C191S). 

While, as has been previously indicated, the plains streams, such as the Red River 
or the Missouri, with their ever-changing banks and bottoms and silt-laden currents, 
present conditions entirely unfavorable to mussels, yet the oxbow or cut-off lakes adja- 
cent to them may offer favorable habitats for several species of mussels (Isely, 1914, and 
Howard, unpublished notes). 

The sand shores of the Great Lakes to a depth of 8 feet are virtually barren of ani- 
mal life (Shelford, 1918, p. 26). Fresh-water mussels are found in these lakes, chiefly, 
it appears, in the shallower bays, where they sometimes manifest a vigorous growth. 
They have not been used commercially to any extent, and probably few possess shells 
of a size and quality rendering them suitable for button manufacture. 

In a biological examination of Lake Michigan in the Traverse Bay region. Ward 
(1896) encountered 9 species of mussels, all of species generally possessing relatively 
thin shells, while Reighard (1894) reported 20 species and subspecies from Lake St. 
Clair, of which the following 8 were described as abundant: 



Pink heel-splitter, Lampsilis alata (Say). 
Thin niggerhead, Quadrula coccinea (Conrad). 
Spike, Unio gibbosus (Barnes). 
Mucket, Lampsilis ligameniina (Lamarck). 



Lampsilis nasutus (Say). 

Black sand-shell, Lampsilis recta (Lamarck). 
Pocketbook, Lampsilis veniricosa (Barnes). 
Floater, A nodonta grandis (Say) . 



A more extensive list of mussels from Lake Erie and the Detroit River is given by 
Walker (191 3), the list including 39 species of 15 genera. Since the great majority of 
the species named are those that normally possess thin and fragile shells, it may be 



lOO BULLETIN OF THE BUREAU OF FISHERIES. 

supposed that the conditions in these waters are not favorable to the production of 
good shells. Certain species are mentioned, however, which, in other regions at least, 
possess shells of commercial quality. Principal among these are the following : 



Maple-leaf, Quadrula lachrymosa (Lea). 
Pimple-back, Quadrula ftustulosa (Lea). 
Pig-toe, Quadrula undaia (Barnes). 



Long solid, Quadrula subrotunda (Lea). 

Hickory-nut, Obovaria ellipns (Lea). 

Black sand-shell, Lampsilis recta sageri (Conrad). 



Clark, collecting on the shores of Lake Erie at Put in Bay, found dead shells all 
dwarfed in form but representing 14 species, of which the more common were as follows: 



Three-ridge, Quadrula undulata. 
Spike, Unio gibbosus. 
Roimd hickory-nut, Obovaria circulus. 
Paper-shell, Lampsilis gracilis. 



Pink heel-splitter, Lampsilis alaia. 
Black sand-shell, Lampsilis recta. 
Fat mucket, Lampsilis luieola. 
Pocketbook, Lampsilis vevtricosa. 



PONDS, SLOUGHS, MARSHES, AND SWAMPS. 

These types of environment are grouped together, since their mussel fauna is gener- 
ally similar. The mussels are thin-shelled as a rule, since light weight is favorable for life 
in mud or soft bottoms and mass is not essential in the absence of current. Some possess 
narrow bodies and keel-like shells that fit them for locomotion through soft soil, and a 
few of the narrow-bodied species, where other conditions are suitable, have relatively 
heavy shells. Such are the pink heel -splitter, Lampsilis alaia, and the white heel- 
splitter, Symphynota complanata. 

The heavier mussels characteristic of rivers are sometimes found in sloughs, but in 
these the characters of flowing and still water are in a measure combined, since strong 
currents may prevail at seasons of high water. Sloughs, as parts of river systems and 
subject to being stocked from them, have mussel fauna to a certain extent related to 
that of the river; that is, the still-water species of the river are to be found in the sloughs. 
Marshes and swamps may have mussels at places where they contain pond or streamlike 
openings. In general the marsh and swamp environment is not favorable to mussels. 

In ponds that are more or less isolated the thin-shelled mussels of the toothless 
type, as Atwdonta grandis (floater) and Anodonioides ferussacianus, are characteristic. 
Lampsilis parOa, one of the tiniest of fresh-water mussels, scarcely exceeding an inch 
in lengfth, is sometimes found in such environments. A characteristic pond-dwelling 
species is the mussel Unio tdralasmus, which will survive in ponds that become dry in 
summer. Examples of this species of mussel have been found alive buried in the bottom 
three months after the water had disappeared on the surface (Isely, 1914, p. 18). 

ARTIFICIAL PONDS AND CANALS. 

Artificial ponds may present a favorable en^iroinnent for many species of fresh- 
water mussels if the water supply is suitable, and some species are likely to become 
accidentally introduced with fish that are brought into the pond. The ponds of the 
Fisheries Biological Station at Fairport, Iowa, are supplied with water pumped from 
the Mississippi River. The first species of mussel to appear in the ponds was the large 
thin-shelled slop-bucket, Anodonta corpidenta, some examples of which had attained a 
length of 3 to 33^ inches when they were first discovered at the expiration of the second 
season of the pond, 17 months (May, 1910, to October, 1911) after the date of introduc- 



FRESH- WATER MUSSELS. lOT 

ing water and fish into the newly excavated pond. Eighteen species which have been 
accidentally introduced are listed on page 165 below. 

Few of these mussels are of commercial value, but it has been attempted to intro- 
duce several useful species by artificial infection upon fish, and success has been at- 
tained with the Lake Pepin mucket, a lacustrine mussel of high commercial value, 
which thrives well in the ponds and has attained a size and quality of shell suitable for 
commercial purposes at the age of 4X years. 

In canals mussels frequently thrive (PI. XI, figs. 3 and 4). A mill race from a well- 
stocked stream seems to present a favorable environment for them. Clark and Wilson 
(1912, pp. 19-22) describe a luxuriant development of mussels in a canal at Fort 
Wayne, Ind., as follows: 

Toward the upper end of the canal , in a place where the bottom was 1 5 feet wide , the mussels were 
counted for a stretch of 10 feet along the canal bed and the following species noted: Quadrula rubiginosa, 
11, Q. cylindrica , i, Q. undulata, 86: Anodonta grandis, 6; Plyclwbranclms phaseolus, i; Lampsilis ligamen- 
iina, 5; L. luleola, 6. The width taken was the total width of the bottom of the canal and was consider- 
ably wider than the space occupied by the mussels. 

About a mile farther down the canal a space of 10 feet square was measured off in the bottom of the 
canal, and the following species were found: Quadrula rubiginosa, 6; Q. undulata, 60, all rather small; 
Pleurobemaclava, i ; Alasmidonta Irurwata, 2; Symphynota complanata, 2; 5. costata, 5; Anodonta grandis, 
1$; Obovaria circulus, 4; Lampsilis ligamentina, 5; L. luteola, i; L. veniricosa, 4. This gave a little over 
one shell per square foot. In igo8, in a square meter of bottom near the Rod and Gun Club, the follow- 
ing species were noted: Quadrula rubiginosa, 9; Q. undulata, 36; Symphynoia complanata, i; Anodonta 
grandis, 17; Obovaria circulus , 11; Lampsilis iris, 2; L. ligamentina, 2; L. luteola, 3, giving a total of 81 
per square meter. In addition to these shells there were many small Sphaeriums, the ground being 
paved with them, 34 Carapelomas, and 33 Pleuroceras. The square meter referred to above repre- 
sents, as nearly as could be judged, an average number rather than either extreme. 

It would appear from a general comparison of the aspect of mussels in lakes, ponds, 
and rivers that the effect of currents or circulation upon the growth of mussels is variable 
according to the relative proportions of organic and mineral foods present. In rivers, 
where the circulation of water is constant, mussels may grow to large size and possess 
thick shells, but when circulation is reduced, as in inclosed bodies of water, the mussels 
may be small and relatively thin-shelled, or they may attain a large size with thin shells 
(suggesting relative deficiency of mineral food), or else, with heavier shells, they may be 
dwarfed in size (suggesting a relative deficiency of organic food). 

BOTTOM. 

Most mussels are normally embedded in the bottom from one-half to three-quarters 
of their bulk.'' That they ma^- thus establish themselves, a firm but not impenetrable 
soil is required. The character of the bottom is, therefore, of especial significance to 
fresh-water mussels, though it has important relations to all bottom-dwelling animals. 
With regard to the bottom, consideration must be given both to its topography and 
to the materials of which it is composed. Major inequalities in topography, such as 
waterfalls and rapids, are discussed elsewhere. Minor inequalities are of importance 
because of the effects upon currents, sedimentation, light conditions, growth of food, 

<• The cases of deep embedding mentioned by Wilson and Dauglade (1913), where they give a depth of i foot or more lor living 
mussel-sin Shell River (p. 15'), and the report of a fisherman of 2 to 3 feet at Lake Bemidji, seem to be cases of "digging in" because 
cf drought. Unio telralasmus (Isely, 1914) and Qnadnita pticata (Howard, 1914) seem to have a remarkable power of resistance 
under these conditions. 



I02 BULLETIN OF THE BUREAU OF FISHERIES. 

and protective conditions; stability of soil is important for the establishment of the 
juveniles, for otherwise they will be overwhelmed. For some species objects for attach- 
ment, to which the byssus of the juvenile may be fastened, may also be necessary. 
Most of the varieties of bottom soil encountered are composed of one of the following 
materials, or of mixtures of two or more of them: Silt, mud, marl, clay, sand, gravel, 
pebbles, cobbles, bowlders, and ledge rock. 

In rivers, sandy bottoms are regions of change comparable to sand-dune areas on 
land where immobile forms are killed. Sand bottoms occur extensively in many rivers 
and they may be veritable deserts. Rivers like the Missouri are devoid of mussels for 
hundreds of miles partly because of a preponderance of bottom of shifting sand. Mussels 
when found on sand bars in rivers are in transit seeking more stable conditions. Although 
comprising regions of instability in rivers where decided currents prevail, bottoms of 
sand may offer more favorable conditions in lakes where they furnish a permanent 
habitat for mussels. 

A greater variety of bottoms favorable for mussels, as well as a more indiscriminate ' 
disposition of them, prevails in rivers than in the other bodies of water considered. In 
many lakes there is a more definite sorting of materials, leading especially to a segregation 
of the finest sediment in the deeper portions of the lake to form a bottom that is very 
soft and generally unsuitable for the Unionidae; mussels possessing much mass would 
sink too deeply and have the gills too much clogged with silt to survive (Headlee and 
Simonton, 1904, p. 176). Where such conditions prevail the mussels are found near 
shore. 

Headlee (1906, p. 315) summarizes observations and experiments in certain lakes 
of Indiana in the following words: 

The work of 1903 and 1904 shows conclusively that the mussels of Winona, Pike, and Center Lakes 
can not exist on the fine black mud bottom — they become choked with mud and apparently smother — 
and that the light-weight forms and the forms exposing great surface in proportion to weight can rest 
on top of comparatively soft mud and can, therefore, live farthest out on the deep-water edge of the 
bed. Because the mussels can not occupy any region where the pure black mud is present, they are 
confined by it to isolated beds and narrow bands of shore line. 

I believe that the whole evidence of the distributional and experimental work of 1903 and 1904 
pointsclearly to the character of the bottom as the great basal influence in the distribution of mussels in 
small lakes generally. 

The species he dealt with were the fat mucket, Lampsilis luteola, Lampsilis stib- 
rostrata, Quadrula rubiginosa, Anodonta grandis, and other small species with light shells. 
While his conclusion accords generally with the observations of the writers in other 
waters, the exclusion of mussels from mud bottoms can not be taken as an invariable 
rule. In the Grand River, at Grand Rapids, Mich., for example, one of the authors has 
observed such a heavy-shelled mussel as the three-ridge, Quadrula utidiUafa, living in 
considerable numbers along with the light floater (Anodonta) in very soft mud. Also, 
in Mississippi Slough, in the Wisconsin lowlands along the Mississippi River opposite 
Homer, Mirm., the blue-point, Quadrula plicala, the pimple-back, Q. pustulosa, and the 
pig-toe, Q. undaia, have been found in considerable numbers on a soft-mud bottom 
along with the heel-splitters, Symphynoia complanata and Lampsilis alaia, and the 
slopbucket, Anodonta corpulenta. 



FRESH-WATER MUSSELS. I03 

Baker (1918, p. 1 17) gives a summary of results of studies of mussels with reference 
to bottom and depth in Oneida Lake, N. Y., in the following words: 

The greatest number of individuals occurred on a clay or sandy-clay bottom. Twice as many 
mussels occurred in water deeper than 6 feet than within the 6-foot contour. These features are 
expressed in Table No. 27, the figures being averages per unit area of 9 square feet. 

TABtB No. 27.— AvER.\GB NnMBER OF Mussels on Bottom. 

Bowlder and gravel bottom 6. 14 

Sand 6. 39 

Clay and sandy clay 13- 00 

Mud 10. 26 

Within 6-foot contour. 7. 84 

Outside 6-foot contour •_ 16. 85 

The above table shows that mussels are more abundant on the mud bottom in deep water (S to 14 
feet) than on sand, gravel, bowlder, or clay in shallow water (i to 6 feet). These are the only studies of 
this character known to tne. 

In that lake one species, Anodonia implicata, is reported from one kind of bottom 
only, in sand between bowlders; while another species, Lampsilis luteola Lamarck, is 
said to be common on all varieties of bottom, except gravel (Baker, 1918, pp. 161, 162). 

Muttkowski (1918) found that sand bottoms marked the favored environments 
of fresh-water mussels in Lake Mendota, Wis. 

In Lake Pepin most of the adult mussels are found on a bottom of gravel or a 
mixture of gravel and sand. Bottoms composed largely of mud but made firm by a 
mixture of sand or gravel or both, yield a good supply of mussels; such areas are of 
much less extent in the lake than bottoms of gravel or gravel and sand. Of 1,397 
juvenile mussels comprising 16 species collected in Lake Pepin in 1914, practically 95 per 
cent were taken on a sand bottom; about 4 per cent, principally Anodonta imbecillis, 
were found on a mud bottom ; and the remaining i per cent on gravel or a mixture of 
sand, gravel, and mud (Shira, report in manuscript). 

In ponds and sloughs there is less choice of bottoms than in lakes, and mud bot- 
toms usually prevail; for such conditions Lampsilis parva, Lampsilis subrostrata, the 
light-shelled Anodontas, and similar species are especially adapted. 

When we consider the relation between various mussel species and the bottom in 
rivers, we find the matter complicated by several considerations. This much, however, 
may be said definitely: No mussels can survive in a shifting bottom, nor upon a bottom 
of solid bare rock. Between the extremes, beginning with clean sand or soft miry silt 
and ending with coarse gravel and bowlders or stiff clay, there is a great variety of 
bottoms utilized to a greater or less extent as habitats for various species of mussels. 

There are, of course, more or less definite relations between bottom and other 
features. Soft, muddy bottom is always associated with a current that is feeble at 
least near the bottom, or with the checking of the current; gravel bottom is usually 
associated with swift current; and clean sand or gravel is associated with clear water. 
Certain of the "mud-loving" mussels, such as the Anodontas, may be really lovers of 
quiet places and their association with mud rather an accident. Some of those supposed 
to be partial to sandy or gravelly bottom may simply prefer clear to turbid water, or 
mav thrive best in a swift current. 



I04 



BULLETIN OF THE BUREAU OF FISHERIES. 



Several species, including most of the Anodontas, Symphynota complanata, Arcidens 
confragosus , and others, are confined chiefly to one sort of bottom. A great many, 
however, seem indifferent to the character of the bottom, provided other conditions are 
favorable. Mussels may also apparently thrive where one would naturally think condi- 
tions unfavorable and where they might not survive if artificially planted. Thus in the 
crescent-shaped bayous along the Kankakee Quadrula undulata, and in sloughs of the 
Mississippi a closely related mussel, both heavy-shelled species, are found thriving on the 
top of deep, soft, silty mud which would not seem stiff enough to bear their weight. 

In the Grand River, Mich., various species of mussels were found upon "clean" 
sand bottoms, but always sparsely. Quadrula undulata and Lampsilis ellipsiforviis and 
venlricosa seemed best adapted to life in accumulations of drift. In sewage and waste- 
polluted waters at Lansing, Mich., Lampsilis ligamentina, venlricosa, and ellipsijormis, 
Quadrula coccinea, Symphynota costala, and Alasmidonta marginata were found in appar- 
ently healthy condition. The Lampsilis ellipsijormis obtained there, of especially large 
size, bore innumerable (but worthless) small pearls. (Coker, unpublished notes.) 

In the Mississippi River near Fairport, Iowa, bottoms of unmixed mud and pure 
sand were found to be much less occupied by many of the species than mixtures of gravel 
and sand or of sand and mud, which supported both a far greater number of individuals 
and a somewhat greater variety of species. The preference for certain bottoms is most 
conspicuous when the proportion of the total catch of mussels found on the favored 
bottoms is viewed in connection -mth the proportions of these bottoms in the total area 
sur\'eyed. Table 5 shows the total number of mussels taken from the different types of 
bottom in a survey of Andalusia Chute, Mississippi River (Howard, report in preparation) : 



Table 5.- 



-Mussels Collected in Survey ov Andalusi.^ Chute, with Reference to Character 

OF Bottom. 



Composition of bottom. 


Approxi- 
mate per- 
centage of 
total area 
of bottom. 


Number oi 
mussels. 


Number oi 
species. 


Mud . . ■....-..^ 


7 
4 

'A 

71 

9 

2 

2 

I 

2 


44 
158 

17 

87 
289 

29 
5 

34 


17 




Mild anH IpHgp rnr-lf ,, , . , , , 




Sand ■,-.■.... .'/; .......if.. :.'-.; l'.; . 








Sand and pebbles .....■.....'!.;... . . 






3 
14 






Pebbles 






Rock 













The niggerhead mussel, Quadrula ebenus, in some streams, at least, is said to show a 
decided preference for firm bottoms, as of gravel or blue clay, but few observations have 
yet been made upon these mussels in streams having considerable areas of clay bottom. 
In the Grand River, Mich., the pink heel-splitter, Lampsilis alata, was found living in a 
ledge of very tough slippery blue clay (Coker, unpubHshed notes). So finn was the clay 
that a mussel could be extricated from it only by the exertion of considerable muscular 
efifort. Several other species of mussel were in the vicinity, but none were embedded in 



FRESH-WATER MUSSEI.S. I05 

the clay except one example of the three-ridge, Quadrula undulata, and that was in a 
spot where the clay was mixed with mud and was distinctly softer. Some spikes, Unio 
gibbosus, were found lying on the blue clay but not embedded; it seemed evident that 
they were unable to penetrate so tough a bottom. 

The character of the soil has an effect upon the amount of materials carried in 
suspension in the water. If the amount is too great, as over soft mud or over extremely 
fine sand, under some conditions the mussel becomes smothered, or having no chance 
to feed, is starved. Too much decomposing organic matter in the soil is said to cause 
enough acidity to attack and erode the shell. For several reasons, therefore, areas of 
rapid silt deposition, or soft-mud bottoms, are quite unfavorable to mussels. Mussels 
are usually found in rivers in places where the bottom is swept clean by the current, 
even though in flood time the water may be heavily laden with silt in suspension. 

The selection by some species of bottoms of gravel, pebbles, and bowlders as most 
favored habitats can readily be understood from the foregoing remarks; but there are 
still other favorable features of rough bottoms. The very stability of the larger-sized 
materials protects the bottom from washing, and may save the mussels from being 
smothered or carried away. It is of advantage to mussels to be surrounded by numerous 
other animals, especially by the smaller ones, which furnish attraction to fishes and thus 
promote the reproduction of the mussel. Many of these small animals live attached to 
stones, thus giving added value to gravelly and rocky bottom. In gravels, too, the 
youngest mussels may be protected through inaccessibility to enemies, and as they 
grow older the resemblance to small pieces of stone among which they lie may be the 
cause of escape from enemies. As previously indicated (p. 97), where bowlder rock or 
cobblestone bottoms occur in regions of rapids, mussels are commonly found abundantly 
and occur over the entire river. 

The following table (6) embodies the experience of several observers regarding the 
preferences exhibited by 62 common species of fresh-water mussels for bottoms of differ- 
ent characters. In view of the intergradation of the several types of bottom and the 
almost unlimited variety of mixtures of sand, gravel, mud, and clay, the classification of 
the bottoms for the purpose of a table must of necessity be rough, and the characterization 
of mixed bottoms may in some cases be affected by the personal equation of the observer. 
Young mussels may have bottom requirements somewhat different from those of adults. 

EXPLANATION OF TABLE 6. 

The letters refer to the experience of the several observers (including the present authors and three 
previous writers), as follows: A, Baker (1898); B, Call (1900); C, Clark; D, Howard; E, Scammon (1906); 
F, Shira; G, Coker. 

The use of large capitals indicates that, according to the observer whose letter is in large capitals, 
a certain type of bottom is preferred by the particular species of mussel. Wherever a small capital is 
used, the observer corresponding to the letter has indicated the type of bottom as favorable for the 
particular species of mussel, but not necessarily preferred to other favorable bottom. 

The observations of Shira refer largely to lake conditions (Lake Pepin, Lake Pokegama, and Caddo 
Lake). 

The observations of Coker refer primarily to shallow rivers (Grand River, Mich.). 

The habitats indicated by Howard are based chiefly on the observed preferences of juvenile mussels 
in rivers, streams, ponds, and slues to the exclusion of true lakes. 



io6 



BULLETIN OF THE BUREAU OF FISHERIES. 



Table 6. — Habitats of Certain Fresh-water Mussels, Classified According to Character 

OF Bottom. 



Scientific name. 


Common name. 


■6 

1 


t 

2 

a 


7j 


•a 
a 
ca 


c 

C 
ca 

■a 


■3 

S 

u 

•a 


■§1 


i 


i 

a 
1 

d 

a 
a 


■6 

1 

a 

> 
a 












D 
DG 




G 


D 






A 
A 

FD 

ABDE 

AEF 

BE 

A 

Bd 




c 




2. Alasmidonta marginata. . , 


Elk-toe 


G 


BCo 




c 
C 




3, Anodonta corpuleiita 


Slop-bucket 




V 

c 

CP 




c 

c 
D 
CD 

c 








Floater 
















Paper-shell 

do 


E 








c 














7. Anodontoides ferussacia- 

nus. 

8. Arcidens confragosus 




G 


GD 






dgC 

C 


G 




c 




Rock pocketbook. 
Fan-shell 


D 
C 

c 

BC 




















10. Dromiis dromas 


Dromedary mussel 




C 




















ABD 








B 

ABEf 

ABEd 

D 

ae 

DF 










Pink heel-splitter. 
Yellow sand-shell. 
Pocketbook 


F 

EGD 
C 
G 

CP 




DP 

nc 

D 




C 
C 


c 


c 




13. Lampsilis anodontoides.. 




























Dg 

DF 
C 
D 

F 
c 


GD 








c 

c 




16. Ldmpsilis fallaciosa 


Slough sand-shell. 


F 




C 

c 


c 




Paper-shell 

Higgin'seye 

Rainbow-shell.. . - 
Paper-shell 


efG 

F 
AG 
EP 
DGP 


P 
D 
C 


D 


F 




ABDEF 


C 


C 














CG 


DG 
P 


G 


c 
C 

C 


A 

EF 
ABd 








CD 






22. Lampsilis lij;amentina 

23. Lampsilis ligamentina 

gibba. 


cfgD 
C 

CF 

c 

F 
D 


cFG 


DG 


c 




Southern mucket . . 




Fatmucket 


CdFG 

AC 
D 

F 

GP 

c 

cFG 

D 

bdFG 

CdfE 
cfdE 

CEGP 
BDEP 


DF 








c 


AEfd 
A 

adef 




c 
c 


















D 




D 
F 
F 




c 




27. Lampsilis purpurata 


Purply 


E 

c" 

D 






Black sand-shell . . 


EFg 








AE 
BED 
dAE 

bd 
Abd 

D 

Ade 
Adef 

B 

d 


c 
c 


r 










30. Lampsilis ventricosa 

31. Margar itaua monodonta . . 

32. Obliquaria rcflexa 


Pocketbook 

Spectacle-case 

Three-horned 

warty-back. 
Hickory-nut 


Cgf 
Cd 

FD 
CFD 

Cdf 
CDf 

F 
D 


D 

Be 
bcdfE 

cdF 

cdF 

CP 

bdeCF 



BD 

Bd 

D 


DP 


G 


c 

c 
c 

c 




D 

D 
D 


D 

DP 




C 

c 


c 




















D 




c 
C 




c 

c 
C 

C 




37. Pleiirobema aesopus 


Bullhead 




















lus. 

39. Quadrula coccinea 

40. Quadrula cylindraca 






cdG 
c 
Dp 

F 
DP 

D 
DP 

C 
DP 

DF 
DG 


Eg 

C 

FGd 

F 


GD 


G 








A 




















D 


D 




C 
C 
c 
C 
c 










42. Quadrula granifera 


Purple warty-back 
















F 
DP 

D 


D 


BDF 

ABFb 

Bd 






r 


44. Quadrula lachrymosa 

45. Quadrula metanevra 

46. Quadrula obliqua 




AEG 


E 
bdCEF 


C 
D 




c 








Ohio River pig-toe 

Blue-point 

Pimple-back 

do 


p 

CD 

fG 
bgG 




C 






F 
c 

CEF 
B 


D 
BDG 


DP 
D 
P 

dE 


G 


C 

c 
C 
c 


AB 

Bf 

ABFdb 

ABd 

Fd 
ABF 
ABdp 

Ad 
Adef 

ABDEF 


cE 




48. Quadrula pustulata 










c 
C 


















Purple warty-back 


G 

DP 

G 

CDGF 

f 


■ ' DF ' 

CG 

GDP 

F 

G 
CDGf 

EP 
C 


bC 
bdF 

G 

CDFG 

F 








c 
C 










D 
G 
G 


D 

D 

DPG 

F 




CO 

c" 

D 


c 

c' 




54. Quadrula undulata 

55. Strophitus edentulus 


Three-ridge 

Squaw-foot 

White heel-splitter 





G 


c 
c 






c 

c 
c 
C 
c 






Fluted shell 


GF 
P 


BCF 

dep 


D 


G 
P 






Adp 
BFde 








C 




60. Truncilla sulcata 


Cat's paw 














F 
F 


G 


C 


BF 

AuEF 








Lady-finger 


DPG 


GP 


DFGe 
















It appears from this and the following table that the preferred bottom for the 
majority of species is mud (but not deep, soft mud, to which type of bottom few species 
are adapted) and gravel, including sand and gravel. Sand ranks next and clay last; 
but few species of mussels exhibit a preference for sand or sandy clay, and only two are 



FRESH-WATER MUSSELS. 



107 



recoided (by one observer) as finding the most favorable environment in a bottom of 
clay unmixed with sand. 

Table 6 may be simplified by reducing the types of bottom to four general classes, 
sand, gravel, mud, and clay, and by eliminating all but the leading commercial species. 
The results are indicated in Table 7 following : 

Table 7. — Preferred Habitats of Leading Economic Fresh-water Mussels, According to 

Character of Bottom. 

IX indicates preference as noted by majority and x by minority of observers.] 



Scientific name. 



Common name. 



Sand.o 



Gravel.ft 



Mud.c 



Clay.* 



Lampsilis anodontoides 

LampsiUs fallaciosa 

Lampsilis recta , 

Lampsilis ligamentina 

Lampsilis lieamentina gibba. 

Lampsilis luteola 

Lampsilis ventricosa 

Obovaria ellipsis 

Plagiola securis 

Quadrula coccinea 

Quadrula ebenus 

Quadrula heros 

Quadrula lachrymosa , 

Quadrula metanevra 

Quadrula obliqua 

Quadrula plicata , 

Quadnda pustulosa , 

Quadrula rubiginosa , 

Quadrula undata 

Quadrula undulata 

Unio crassidens 

Unio gibbosus 



Yellow sand-shell . . 
Slough sand-shell. . 
Black sand -shell. . . 

Mucket 

Southern mucket. . 

Fat mucket 

Pocketbook 

Hickory-nut 

Butterfly 

Flat niggerhead 

Niggerhead 

Washboard 

Maple-leaf 

Monkey-face 

Ohio River pig-toe. 

Blue-point 

Pimple-back 



X 
X 
X 



X 

X 
X 



X 
X 



X 
X 

X 



X 

X 



Pig-toe 

Three-ridge. . . . 
Elephant's ear. 
Lady-finger. . . 



X 

x" 



X 

x 

X 
X 



a Sand alone. 

t> Including sand and gravel, mud and gravel, and rocks. 



^ Mud alone. 

^ Including sand and clay, mud and clay. 



DEPTH. 

The distribution of many animdis of the water is known to be influenced by depth, 
the effect of which may be felt, among other ways, through pressure, light, temperature, 
dissolved gases, and freedom from wave action, or exposure thereto. In an indirect way, 
too, the effect of depth is experienced by any animal through the influence of these 
conditions upon food and enemies. 

The increase of pressure is approximately i atmosphere for each lo meters (33 
feet) in depth, but fresh-water mussels are, so far as known, restricted to shallow waters 
where pressures must be insignificant. The Sphseriids are the only mollusks found 
below the 25-meter line in Lake Michigan (Shelford, 1913). Maury (1916, p. 32), (see 
Baker, 1918, p. 155), reporting the results of dredging in Cayuga Lake, N. Y., says: 
"These dredgings proved conclusively that MoUusca after 25 feet become very scarce. 
* * * In the greater depths no signs of Mollusca or of plants were found." In 
clear water minor depths do not markedly affect the light, but if the water is turbid, a 
common condition in the environment of fresh-water mussels, the penetration of light 
is very much diminished (see p. 114), and mussels if affected by light may, therefore, 
be expected to live at greater depths in clear lakes than in turbid streams. Temperature 
changes due to depth alone are so inconsiderable for shallow water as doubtless to have 
little effect upon the distribution of mussels, except where freezing to the bottom may 
occur. 

9745°— 21 3 



Io8 BULLETIN OF THE BUREAU OF FISHERIES. 

The depth of water below which waves would reach them is apparently a factor in 
determining the habitat of many species of mussels in lakes (Headlee, 1906, p. 308 — 
Winona Lake; Muttkowski, 1918 — Lake Mendota). In large bodies of water like Lake 
Michigan the action of the waves is said to extend to 8 meters below the surface. The 
zone of wave action is a region in lakes comparable to the rapids and riffles of streams, 
where there is maximum circulation and aeration and a solid bottom suitable for 
such mussels as can withstand the violent action of waves and undertow currents. 
The species occupying this zone are given by Headlee for Winona Lake as the spike, 
Unio gibbosus, and the fat mucket, L. luteola. Baker (1916) says of this habitat in 
Oneida Lake : 

The shore may be free from vegetation. It receives the full force of the winds and waves from the 
open lake. The water is from i to 3 feet in depth and the bottom is heavily and thickly covered with 
stones and bowlders, many of the latter being of large size. Animal life is abundant, the clams living 
between the stones and on the sand between the stones. 

The mussels he reported are as follows: Elliptio complanaius, common; LampsUis 
luteola, rare; LampsUis radiata, common; LampsUis iris, rare; Margaritana m,argari- 
tijera, rare; Anodonta caiaracta, common; Anodonta iviplicata, coramoia; Anodonta 
grandis, common; Strophitus edentulus, rare. Some of these are very thin shelled and 
doubtless survive the force of the waves only through the protection afforded by the 
large rocks. No doubt the thorough aeration of the water, resulting from wave action, 
is a favorable factor in this zone 

On the shores of Lake Pepin one of the authors has often picked up live mussels 
that had been thrown up by heavy wave action. The mussels thus most frequently 
encountered were Unio gibbosus, LampsUis alata, Anodonta corpulenta, Strophitus eden- 
tulus, LampsUis vcntricosa, and LampsUis luteola in about the order named. They were 
usually immature examples. Occasionally after a stomi had subsided one could see 
mussels that had not been entirely stranded on the beach near shore and in the act of 
making their way back' again into deeper water. Headlee and Simonton (1904, p. 175) 
recorded similar observations. 

While the data available are sufficient only to suggest how depth may affect the 
haoitat selection of mussels, it is of interest to note some of the observations on this 
relation. A maximum depth of 22 feet for mussels in Winona Lake is given by Headlee 
(1906), who ascribes the control of distribution to bottom characters chiefly. Baker 
(191 8) found that in Oneida Lake twice as many mussels occurred in water deeper than 
6 feet as within the 6-foot contour. (See quotation, p. 103, above.) He records three 
species as limited to a depth of i K to 8 feet, three as living at varying depths between 
i}4 and 18 feet, and one subspecies as occurring only between 8 and 18 feet, the greatest 
depth which he explored. He reports an interesting case of bathymetric distribution of 
two races, LampsUis radiata, occurring at i K to 3 feet, and a subspecies, LampsUis radiata 
oneidensis, living only at 8 to 18 feet, the two forms showing a distinct difference in 
habitat. For Lake Mendota the optimum depth for mussels of the genera Anodonta 
and Lampsilis is given as from 2 to 3 meters (6 to 10 feet) (Muttkowski, 191 8, p. 477); 
they were, however, found abundantly between 3 and 5 meters and rarely at greater 
depths than 7 meters (23 feet). 

Wilson and Dangiade (1914), in reporting a reconnoissance of mussel resources in 
Minnesota waters, give depths of the lakes, but without detailed data on the distribution 



FRESH-WATER MUSSELS. IO9 

of the mussels. In Lake Maxinkuckee, Evemiann and Clark (1918, p. 255) say: "Mussels 
are to be found almost anywhere iii water 2 to 5 or 6 feet deep where the bottom is 
more or less sandy or marly." Headlee (1906, p. 306) found that the mussel zone 
generally extended from the shore line to where the bottom changes from sand, gravel, 
or marl to very soft mud, a region in Winona Lake covered by from 4 inches to 9 feet 
of water. He did find, however, some mussels on sandy bottom in 22 feet of water. 
He made some experiments in retaining mussels at various depths and in a crate placed 
in 85 feet of water; only i of 10 specimens died in six days of exposure. After 12 days 
several specimens were found badly choked with mud. 

In Lake Pepin mussels are plentifully found at depths ranging from 8 to 20 feet, 
but the majority are taken at depths ranging from 12 to 1 8 feet. Relative to the juvenile 
mussels, out of a total of 1,397 collected in 1914, 1,283, or 9i-8 per cent, were taken ata 
depth of 3 to 8 feet; 2.6 per cent at 8 to 12 feet; 2.3 per cent at 12 to 16 feet; 0.4 per 
cent at 1 6 to 20 feet ; and 2.9 per cent at 20 to 25 feet. A . imhecillis was the only juvenile 
found in any abundance at a depth greater than 15 feet, and 41 of the 79 individuals of 
this species collected were taken at 25 feet (Shira, report in manuscript). 

A marked distribution with regard to depth has been observed in the artificial 
ponds at Fairport, Iowa. Here the species, Lampsilis luleola, is seldom found below a 
depth of 3 feet. When held in crates below this depth it does not thrive, although in its 
natural habitat. Lake Pepin, this species is abundant at a depth of 8 to 20 feet and has 
been taken at a depth of 25 feet. 

In rivers and smaller streams mussels seem to be found commonly at lesser depths 
than in lakes, but unfortunately we have very few reports of observations in the deeper 
parts of large rivers. In the Illinois River, Danglade (191 4) mentions a small bed 2 to 3 
acres in extent above the mouth of Spoon River, where the bottom was of mud, the 
current about 2 miles per hour, and the depth of water 8 feet. At Chillicothe he found 
a good bed at a depth of 1 2 to 1 5 feet. The survey of Andalusia Chute, Mississippi River 
(Howard, report in preparation), carried on during relatively high-water stages in 1915, 
revealed no mussels in the deeper portion of the river over 12 feet in depth, and the 
greater number of mussels were found at depths less than 10 feet. Local informants at 
Madison, Ark., stated that the niggerhead, Quadrida ebenus, was found in water 20 to 50 
feet deep; it was also said that in flood season it was captured from a depth of 75 feet. 
There has been no opportunity, however, to verify these statements. 

With regard to a collection of 183 juveniles of the Quadrula group from 12 stations 
in the Mississippi River, Howard (1914, p. 34) reported depths from o to 8 feet. Wilson 
and Clark (191 4) reported a rich find (19 species) in the Rock Castle River off the Cum- 
berland, in water having a maximum' depth of iK feet. In the Grand River, Mich., 
the senior author has found mussels (muckets, Lampsilis ligamentina, three-ridge, 
Quadrula undulata, and others) in conspicuous abundance in swift water less than a foot 
in depth. Boepple (Boepple and Coker, 191 2) found mussels abundant and of fine 
commercial quality in water from i to 3 feet in depth in the Holston and Clinch Rivers 
of Tennessee. In Caddo Lake, Tex., Shira (1913) found an abundance of mussels in 
4 to 8 inches of water, and in many places there was scarcely enough water to cover the 
shells. This lake was very shallow over large areas. In fact, mussels are frequently 
found in very shallow water where the conditions of the bed of the stream and other 



no BULLETIN OF THE BUREAU OF FISHERIES. 

factors are favorable. In various parts of the country considerable commercial quan- 
tities of mussels are collected by hand from shallow waters. At one such place, Lyons, 
Mich., the mucket, Lampsilis ligamentina comprised 80 per cent of the collection, although 
the three-ridge, Quadrula undiUala, the pocketbook, Lampsilis ventricosa, the spike, 
Unio gihbosus, and the black sand-shell, Lampsilis recta, were quite common. Among 
other species that were frequently found in very shallow water (i to 2 feet in depth) in 
that stream were the following: Lampsilis luteola, iris, and ellipsijormis, Quadrula 
coccinea and rubiginosa, Sirophitus edentulus, Symphynoia compressa and costata, Alasmi- 
donta marginata, Anodontoides jerussacianus, and Anodonta grandis. In fact, the only 
species that were not found in water less than 6 feet in depth in the Grand River were 
the three-homed warty-back, Obliquaria reflexa, the hickory-nut, Obovaria ellipsis, the 
deer-toe, Plagiola elegans, and the white heel-splitter, Symphynota complanata. 

LIGHT. 

The small floater, A nodonla imbecillis Say, in sunlight will draw in its siphons when 
a shadow passes over. Wenrick (1916) has demonstrated experimentally mth measured 
illumination, that a fresh-water mussel, Anodonta cataracta Say, is very sensitive to 
decrease in intensity of light. Observations in the Washington laboratory indicate that 
the yellow sand-shell, Lampsilis anodontoides, will close when a black cloth is placed 
over the aquarium, but will open when exposed either to daylight or to the light of a 
bright electric lamp. These reactions may be for protection of the animal from approach- 
ing enemies, but it is probable also that the distribution of mussels is largely influenced 
by light conditions. Mussels are seldom found in vegetation which is dense enough to 
exclude the light to a great extent. This is especially true with regard to plants like the 
water lily which have floating leaves. Some relations to vegetation are brought out 
in a study of the habitats in Oneida Lake (Baker, 191 6). 

An exceptional case is reported by Wilson and Danglade (191 4, p. 15) where the 
mussels were found in densest aggregation submerged deeply in the bottom and below 
a covering of vegetation. Their account is of sufficient interest to be quoted in full : 

The bottom of the river where these shells are obtained is covered with algae and water weeds to the 
depth of 12 to 18 inches, and the thicker the vegetation the more plentiful the mussels beneath it. Two 
men were actively working the Shell River at Twin Lakes near Menahga at the time of our visit, and we 
watched them rake off the algae and weeds and then dig into the underlying gravel and sand for the 
mussels. The latter are often buried to the depth of a foot or more. This is, at the least, a novel con- 
dition and one which, so far as is known, has not been reported from any other locality. 

Certain species of mussels, the mucket, pocketbook, black sand-shell, and others 
are sometimes pink-nacred and sometimes white-nacred, and with the two former, at 
least, the outside covering of the shell has a reddish cast in pink-nacred examples. 
With such species it is a matter of common observation that pink-nacred shells and 
brightly colored exteriors are more frequently found in shallow clear water where the 
mussels are exposed to bright light." Thus the black sand-shells of the upper part of the 
Grand River, Mich., have a deep purple nacre, while white shells of the same species 
predominate in the more turbid Mississippi. The spike, Unio gibbosus, is usually purple- 
nacred, but uncommon examples that are nearly white are found in turbid rivers. Clark 

" Grier (1920a) presents the result of an extensive study of the nacreous color of mussels. He notes a tendency to lighter of 
bluish nacreous color in the lower portion of stream courses- He has evidence of some correlation between color and sex. 



FRESH- WATER MUSSELS. • III 

and Wilson (191 2) describe the Maumee River as ratiier muddy most of the time, and it 
is interesting to find that they report that two-thirds of the spikes, Unio gibbosus, in that 
river were white-nacred and that the black sand-shells were usually white-nacred. 

The reputed migration of certain mussels toward shore in time of flood may be an 
accommodation to light conditions associated with turbidity of water under such con- 
ditions. We have virtually no data on the distribution of mussels with respect to 
permanently shaded areas or with regard to the reactions to daily changes in light. 

CURRENT. 

The luxuriant development of certain mussels in streams where the current is 
strong, in contrast with their growth in sluggish portions of rivers and lakes, bears 
witness to the significance of current as a favorable factor of environment for fresh- 
water mussels. Current is a characteristic feature of streams, and the rate of flow is 
largely determined by the gradient of the channel. Currents producing a circulation 
of water occur also in lakes, where they are caused chiefly by wind and to a less extent 
by changes of temperature. In some lakes the circulation extends from top to bottom, 
but in small deep lakes only a partial surface circulation commonly prevails (Birge and 
Juday, 191 1 ). Undertow currents are also developed where there is wave action, and 
under some conditions convection currents must exist in natural bodies of water, but 
we have little data on this. 

Shelford (1913) emphasizes the relation of water animals to current as follows: 

The distribution of dissolved salts and gases is dependent upon the circulation of the water, as 
their diffusion is too slow to keep them evenly distributed. The water of streams has been found to 
be supersaturated with oxygen [citing Birge and Juday, 1911]. Oxygen is taken up by water near the 
surface. Nitrogen and carbon dioxide are produced especially near the bottom, and if the water did 
not circulate they would be too abundant in some places and deficient in others for animals to live 
(p. 60). * * » 

The current in streams differs from that in lakes in that it is for the most part in one direction 
while the lake currents often alternate. There are backward flows and eddies at various points in 
streams in front of and behind every object encotintered in the current. As we pass across a stream 
we find the current swiftest near the surface in the middle and least swift at the bottom near the sides 
(p. 61). * * * 

The factors of greatest importance in governing the distribution of animals in streams are current 
and kind of bottom. They influence carbon dioxide, light, oxygen content, vegetation, etc. (p. 66). 

Since mussels are bottom dwellers and largely stationary in habit, one can appreciate 
how dependent they must be upon circulation of the water to bring renewed supplies 
of organic food, mineral matter in solution, and oxygen, and to remove the poisonous 
products of metabolism that are produced in their own bodies and in those of other 
organisms living about. Mussels, of course, cause by their respirative currents cir- 
culation of the water immediately about them, but this is not sufficient to prevent 
an early exhaustion of food supply unless broader currents prevail. 

It must be emphasized, too, that flowing water carries more matter in suspension 
than still water. It has been seen (p. 91) that the food of mussels consists to a con- 
siderable extent of the finely divided solid matter; but such materials, however abun- 
dant on the bottom, are not available to the mussel imtil they are taken up in the water 
and carried to the mussel. The effects of the current, then, both in lifting solid matter 
from the bottom and in holding it in suspension play a foremost part in its relation 



112 BULLETIN OF THE BUREAU OF FISHERIES. 

to the welfare of mussels. The power of water to move solid matter on the bottom 
increases very rapidly with the rate of flow. 

The capacity of water to move solid matter from a condition of rest on the bottom of a stream 
varies with the sixth power of the velocity of the stream. If the velocity is doubled, the increase 
in the force which is capable of putting the particle in motion is multiplied 64 times. (New York 
report of Metropolitan Sewerage Commission, 1912, p. 41.) 

Fish frequent areas near the current but maintain themselves in eddies or in places 
where the current is relatively slack, as at the bottom and near the shores (vShelford, 1913). 
In view of the essential part that fish play in the distribution of mussels, the habits 
of the fish may be a very significant factor in the distribution of mussels with reference 
to current. It has been suggested by Evermann and Clark (1918, p. 252) that currents 
may promote the reproduction of mussels by making fertilization of the egg more 
certain and by decreasing the chance for inbreeding through the conveyance of sperm 
from mussels farther upstream. In still waters the chance for fertilization of eggs 
may be less favorable. 

The relations of mussels to temperature have not been fully investigated, but it 
seems certain that flowing water must protect mussels from excessively high tempera- 
tures and thus permit many species to live in much shallower water in streams than 
in ponds or lakes. 

The tendency of mussels to locate apart from the main channel and nearer the 
banks of the streams has previously been mentioned (p. 97). While this distribution 
may be partly due to the fact that there the full force of the current is avoided while 
many of its benefits are received, nevertheless it must not be overlooked that many 
species of mussels thrive in rapid shallow streams and that such regions of swift water 
in the Mississippi River, as the fomier "rapids" at Keokuk or the existing "rapids" 
above Davenport, have been among the most prolific mussel grounds of the entire river. 
In these circumstances, however, the rocky nature of the bottom affords the mussels 
protection against some' effects of the current. Evidently the barrenness of the main 
channel in most cases is due rather to the nature of the bottom combined with the force 
of flow than to the strength of current alone. 

On page 99 there have been listed the species of mussels which are characteristic 
of lakes and ponds, regions of comparatively still water. The more common mussels 
of rivers may be classified according to apparent adaptation to sluggish water, strong 
current, and rapids (Table 8). These general comments should be made: In a firm 
bottom, such as furnishes good anchorage, a mussel may dwell in a current swifter 
than is characteristic of its common habitats; where rocks furnish shelter, mussels 
below them may be in rather slow water despite the current around them; deep water 
may be fairly sluggish under a swift surface current. 

EXPLANATION OF TABLE 8. 

The symbols are those used in Table 6, C representing Clark; D, Howard; F, Shira; and G, Coker. 
The large capital denotes preference in the opinion of the observer, for a particular condition of current. 
The small capital denotes that the condition is favorable but not, so far as is known, preferred to other 
conditions. When no large capital occurs on a line, no preference is indicated; and when a particular 
letter appears in small capital throughout a line, the observer denoted by the letter has no evidence 
upon which to base an opinion of discrimination on the part of the particular mussel between the 
different conditions of current regarded as favorable. 



FRESH-WATER MUSSElvS. II3 

Table 8. — Classification of Common Fresh-water Mussels in Relation to Current. 



Scientific name. 



Common name. 



Little or no 
current. 



Fair or good 
current. 



Stn»igor 
swift cur- 
rent. 



Alasmidonta calceola 

Alasmidonta raarginata 

Anodonta corpulenta 

Anodonta grandis 

Anodonta imbecillis 

Anodonta suborbiculata . . . . 
Anodontoides ferussacianus. , 

Arcidens confragosus 

Cyprogenia irrorata 

Dromiis dromas 

Hemilastenia ambigua 

Lampsilis alata. 

Lampsilis anodontoides 

Lampsilis capax 

Lampsilis ellipsiforniis 

Lampsilis fallaciosa 

Lampsilis glans 

Lampsilis gracilis ._ 

Lampsilis hig&insii 

Lampsilis iris 

Lampsilis laevissima 

Lampsilis liganieotiua . ._ 

I^ampsilis ligamenlina gibba 

Lampsilis luteola 

Lampsilis muUiradiata 

Lampsilis pan'a 

Lampsilis purpurata 

Lampsilis recta 

Lampsilis subrostrata 

Ivampsilis ventricosa 

Margaritana monodonta 

Obliquaria rcilexa 

Obovana ellipsis' 

Plagiola donaciformis 

Plagiola elegans 

Plagiola securis 

Pleurobema aesopus 

Plychobranchus phaseolus. . 

Quadrula coccinea 

Quadnila cylindrica 

Quadnila cbemis 

Quadrula granitera 

Quadrula heros 

Quadrula lachr>Tnosa 

Quadrula metanevra 

Quadrula ol^liqua 

Quadrula plicata 

Quadnila pustulata 

Quadrula pustulosa 

Quadrula rubijcinosa 

Quadrula trapezoides 

Quadrula tuberculata 

Quadrula undata 

Quadrula undulata 

Strophitus edentulus 

Symphynota complanata, . . 
Symphynota coraprcssa. . . . 

Symphynota costata 

Tritogonia tuberculata 

TrunciUa sulcata 

Unio crassidens 

Unio gibbosus 



Slipper-shell. 

Elk-toe 

Slop-bucket . . 

Floater 

Paper-shell... 
....do 



CDF. 
CDF. 
CDF. 
CD... 



Rock pocketbook . . . 

Fan-shell 

Dromedary mussel. 



CG 

CDF. 



Pink heel-splitter. 
Yellow sand-shell. 
Fat pocketbook . . . 



D 

cDFG. 



Slough sand-shell. 



Paper-shell. 

Higgin's eye 

Rainbow-shell. . . . 

Paper-shell 

Mucket 

Southern mucket. 
Fat mucket 



cD 

CG 

DF.... 

CDFG.' 



cgD.. 
CDF. 



CDFG. 



Purply 

Black sand-shell. 



CDF. 
cF... 



Pocketbook 

Spectacle-case 

Three-homed warty-back. 
Hickory-nut 



CD... 

CFGD. 



Deer-toe 

Butterfly 

Bullhead 

Kidney-shell 

Flat niggerhead 

Rabbit's-foot 

Niggerhead 

Purple warty-back. 

Washboard 

Maple-leaf 

Monkey-face 

Ohio River pig-toe. 

Blue-point 

Fimple-back 

do 



DP. . 

DF. 
F 



Bank-climber 

Purple warty-back. 

Pig-toe 

Three-ridge 

Squaw-foot 

Wliite heel-splitter. 



cF. 



DP.... 
cgD... 
cgDF. 
CPG. . . 



Fluted shell.... 

Buckhom 

Cat's paw 

Elephant's ear. 
Lady-finger 



cdgF. 



CG. 
C... 
ct.. 

CG. . 



CD... 
CF 

CDF. 
C... 
CG... 
CF... 

c 

CDF.. 

CDF. 
CG... 

F 

CF... 

C... 

CFG. . 
C 



CP 

CDFG. 

c 

CD KG. . 

CD 

CFGd.. 

CDFG 

CDF... 

CFG... 

CDF... 

CDF... 

C 

CDg... 

CF 

CFd. . . 

CD 

CDF. . . 
CFG. . . 
CDF... 

C 

CDF... 
CDF... 
CFGd.. 
CDG... 

CF 

CG 

CFd... 
CFGd.. 

CDG . . . . 

CDF... 



CF... 
CFd.. 
C... 
CDF. 

CDG. . 



DG 



DG 
c 



dG 



D 

DG 



Dg 



WATER CONTENT. 



The matter that is carried in all natural waters in varying quantities and proportions 
consists of suspended matter, botli dead and living, minerals and other ordinarily solid 
substances in solution, and dissolved gases. All of these classes of substances are utilized 
by fresh-water mussels in one way or another, and the quantity of any of them in the 
water has a direct bearing upon the suitability of waters for mussels. 



114 BULLETIN OF THE BUREAU OF FISHERIES. 

SUSPENDED MATTER. 

The solids carried in suspension by water consist of mineral and organic substances. 
The particles of mineral matter brought in by surface drainage or derived from bottom 
and shores, apart from that which is in solution, range in size from coarse to very minute. 
The carrying power of the water varies with the sixth power of the velocity, although 
in the case of the most minutely divided substances other factors than rate of flow 
come into play. 

Mussels are afifected in various ways by the matter in suspension. It has been 
reported that some mussels stop feeding when the water is excessively turbid, as after 
a storm. In this way they would avoid taking into their stomachs large amounts of 
indigestible mineral. They have, however, the power of ejecting undesirable matter; 
this may enable them to continue feeding even though the water is moderately turbid 
In streams like the Mississippi, mussels could hardly survdve without feeding during 
the long periods of turbidity that prevail. Excessive precipitation of silt may smother 
or even bury the mussel (Headlee and Simonton, 1904, p. 176). The turbidity of water 
over deeper beds materially restricts the amount of light reaching the mussel, and it is 
possible that this has an untoward effect. Data regarding the turbidity of several 
streams are given in Table 9, page 116. The turbidity of representative mussel-producing 
streams varies from 37 to 188, except that the Des Moines River at Keosauqua has a 
turbidity rating of 542 — a striking exception. The Missouri and Red Rivers (non- 
productive) and portions of the Mississippi River which do not yield commercial mussels 
have turbidity ratings from 556 to 1,931. 

Organic materials, both living and dead, are abundantly suspended in most natural 
waters, and form a large part of the food of mussels. (See p. 91.) The living bodies 
are the microscopic plants and animals which make up what is called the plankton. 
The dead organic materials are the remains or fragments of plants and animals in a 
state of decomposition, and such also form a part of the food supply. 

Some of the plankton originates in the lake or stream in which the mussels are 
living. Another and perhaps the greater part is brought in by the tributary streams. 
Similar statements may be made regarding the dead organic matter, with the addition 
that some of this may be brought in by surface drainage from the bordering lands. 

MINERALS IN SOLUTION. 

To what extent mussels derive the mineral matter necessary for the sustenance of 
life and the formation of shells directly from the water or through the solid food con- 
sumed can not be said, but even that part which is derived from solid food must have 
been obtained by the smaller organism from the water or the soil. Churchill (191 5 and 
1916), from experiments conducted at the Fairport Station, has shown that fresh-water 
mussels possess the ability to make use of nutriment which is in solution in the water. 
WhUe he demonstrated this for such nutritive substances as fat, protein, and starch, 
there are yet wanting, as he has pointed out, analyses of the natural water in which 
mussels live to prove that such organic substances are present in the waters in quantities 
sufficient to play an important part in the nutrition of mussels. There are, however, 
abundant analyses to prove the presence of dissolved minerals. 

The requirements of mussels in mineral food may be ascertained by analysis of 
the soft bodies and shells, Such analysis shows that while the shell is about 95 per cent 



FRESH- WATER MUSSELS. II5 

calcium carbonate, and 3K per cent organic matter, it also contains other minerals in 
very small proportions, less than i per cent each, such as silica, manganese, iron, alumi- 
num, and phosphoric acid. It does not follow that because these minerals, other than 
calcium, occur in minute proportions, they are any the less essential to the welfare of 
the mussel; iron forms a very small proportion of the human body, but man can not 
live without it. So these minerals may, then, be just as essential to the formation of 
good shell as calcium, but with the possible exception of manganese it is probable that 
all natural waters contain a sufficient quantity of the minerals to satisfy the needs of 
mussels. Nevertheless an interesting and important problem may be found in a com- 
parative study of the mineral content of different waters which yield shells of diverse 
qualities. It is even possible that an excessive proportion of certain minerals in water 
tends to the formation of shells that are brittle, discolored, or otherwise inferior. 

The sundried meats of mussels from the Mississippi River when analyzed have 
been found to contain, besides moisture (about 7.6 per cent), protein (calculated from 
nitrogen), 44 per cent; glycogen, about 9 per cent, ether extract (presumably fats), 
a little less than 3 per cent; and undetermined organic material, 13 per cent. The 
remainder is mineral matter (chiefly phosphoric acid), 9 per cent; calcium (calcium 
oxide), 8 per cent; silica, 3>^ per cent; manganese, about one-half of i per cent; and 
such other minerals in small proportions as sodium, potassium, iron, and magnesium 
(Coker, 191 9, p. 62, analysis by U. S. Bureau of Chemistry). 

As previously indicated, nearly all natural waters, at least those fed largely with 
surface drainage, probably contain certain quantities of the required minerals, but it 
would be going beyond the bounds of present knowledge to say whether or not the 
abundant growth of mussels in certain streams and the variable qualities of shells 
produced in different streams are related to the proportions of minerals present other 
than calcium. Certain it is that a deficiency of lime is very unfavorable. The soft 
waters of the Atlantic slope support very few mussels and these are small in size and 
possess thin shells which are usually badly eroded. The thinness of the shells is asso- 
ciated with the deficiency of calcium in the water, and the erosion is an indirect result 
of the same cause, since the free carbonic acid, which attacks and consumes the shells 
wherever the protective horny covering has been broken by abrasion, would, in harder 
waters, be combined with the calcium in solution to form the bicarbonate. 

Circulation, of course, plays a great part in making available to mussels the dissolved 
content of the water. It may be due not so much to low calcium content as to inadequate 
circulation that small lakes and ponds in States of the Middle West generally yield 
mussels with thin or dwarfed shells. 

The waters of many streams of the United States have been subjected to analysis 
by the United States Geological Survey (Dole, 1909). The summarized analyses for 
several streams, or parts of streams, productive of mussel resources, and for 10 others 
that are not productive of commercial shells, are given in Table 9 below. It appears 
that, within broad limits, the variations in content of silica, iron, magnesium, sodium, 
and potassium are not significant as affecting productiveness (unless, as may be the 
case, the quality of the shell produced is affected) . Particular attention may be directed 
to the columns of turbidity, calcium, carbonate radicle, and nitrate radicle. The 
nonproductive streams, or parts of streams, listed are generally either very high in 
turbidity or very low in calcium, bicarbonate, and nitrate. The Shenandoah, among 



ii6 



BULLETIN OF THE BUREAU OF FISHERIES. 



nonproductive streams, is an interesting exception. So far as can be seen, its analysis 
conforms essentially to the standard of productiveness in mussels as revealed by streams 
of the Mississippi Basin. It is possible, then, that the Shenandoah, and perhaps a few 
other streams of the Atlantic or Pacific slopes, might support fresh-water mussels of 
commercial value should the proper species be introduced. 



Table 9.- 



-Contents of Waters op Certain Productive Mussel Streams and Other Nonpro- 
ductive Streams." 



PRODUCTIVIv RIVERS. 

Wabash, Vincenues. Ind 

lUinois, La Salic. Ill 

niinois, Kamps\'ille, 111 

Fox. Ottawa. HI 

San^ramon. Springfield. Ill 

Cumberland. Nashville, Tenn. . 

Cumberland, Kuttawa. Ky 

Des Moines, Keosauqua, Iowa. . 

Grand, Grand Rapids, Mich 

Cedar. Cedar Rapids. Iowa 

Maumee. Toledo, Ohio 

Mississippi, !Moline, 111 

Mississippi, Quincy, 111 

NOKPRODUCTIVE RIVER.S, 

James, Richmond, Va 

Potomac, Cimiberland, Md 

Wateree, Camden, S. C 

Shenandoah. MiUville, W, Va. . 

Mississippi, Chester, 111 

Mississippi, Memphis, Tenn 

Red, Shrcveport, La 

Missouri, Ruegs, Mo 

Savannah, Augusta, Ga 

Hudson. Hudson, N. Y 

Cape Fear, Wilmington, N. C. . 



Turbidity. 



17a 
159 
188 
94 
74 
136 
176 
54» 
37 
64 
'43 
117 
173 



90 

23 

359 

31 

858 

5S6 

790 

1. 931 

173 

13 

73 



Suspended 
matter. 



193 
■36 
14s 

87 

39 
94 
i6s 
643 
43 
6r 

113 

106 
119 



71 
29 

314 

39 
634 
519 
870 
,890 
143 
16 



Coeificient 
of fineness. 



.80 
.80 
1. 30 
.80 

•74 

• 93 

I.09 
1. 61 

-97 
•95 



.96 
I. 59 

•79 
1.64 



•97 



•77 
1.36 



Total 
iron (Fe) 



SiUca 
(SiOs). 


Iron (Fe). 


li.O 


0. 34 


IJ.O 


. 21 


13.0 


•"7 


II^O 


.ao 


16.0 


■i' 


20.0 


.43 


18.0 


.30 


33.0 


.36 


14.0 


•07 


14.0 


.09 


17.0 


.37 


16.0 


•59 


18.0 


.46 


18.0 


.5 


8.3 


.14 


25.0 


.38 


iS^o 


.08 


33.0 


•39 


34.0 


.61 


30.0 


1.1 


39.0 


.51 


33- 


.44 


II. 


■IS 


9.9 


•78 



Calcium 

(Ca). 



61.0 
so.o 
47-0 
60.0 

53.0 
26.0 
28.0 
58.0 
56.0 
48.0 
57.0 
33-0 
36-0 



14.0 
24.0 
6.3 
33.0 
44.0 
36.0 

74- O 
52-0 
5-7 

2I>0 
S-O 



Magne- 
sium 
(Mg). 



23-0 
22.0 
20.0 
33- O 
24>0 
3-6 

4-3 

21-0 

19.0 
16.0 

16.0 

13- o 

16.0 



3-0 
4-6 



16.0 
12.0 



3-8 



Sodium 
and potas- 
sium 

(Na-f-K). 



Carbonate 

radicle 
(CO3). 



Bicarbo- 
nate radicle 
{HCO3). 



Sulphate 
radicle 
(SOO- 



Nitrate 
radicle 
CNO3). 



Chlorine 
(CI). 



ToUl 

dissolved 

solids. 



PRODUCTIVE RIVERS. 

Wabash, Vincennes, Ind 

Illinois, La Salle, lU 

Illinois, Kampsville, 111 

Fox, Ottawa, 111 

Sangamon, Springfield, 111 

Cumberland, Nashville, Tenn... 

Cumberland. Kuttawa, Ky 

Des Moines, Keosauqua, Iowa . . 
Grand, Grand Rapids, Mich. . . . 

Cedar, Cedar Rapids, Iowa 

Maumee, Toledo, Ohio 

Mississippi, Moline, 111 

Mississippi, Quincy. Ill 

NONPRODUCTIVE RIVERS, 

James, Richmond. Va 

Potomac, Cumberland, Md 

Wateree. Camden, S. C 

Shenandoah, MiUville. W. Va. . . 

Mississippi, Chester. Ill 

Mississippi, Memphis, Tenn 

Red, Shreveport, La 

Missouri, Ruegg, Mo 

Savannah, Augxista. Ga 

Hudson, Hudson, N. Y 

Cape Fear, Wilmington, N. C. . . 



35-0 
16.0 
18.0 
14.0 
16.0 
9.6 
7.8 

17-0 
10. o 
12.0 

24>0 

10. o 



6.7 

9-0 

8.4 

6.7 

21.0 
19.0 
90.0 

36-0 
12.0 
7-9 
7.2 



8.5 



4.6 



330 


55^o 


303 


50.0 


303 


43.0 


27s 


61.0 


347 


37- 


93 


14.0 


100 


9-7 


3l5 


71. 


314 


33^o 


309 


30.0 


173 


48.0 


IS3 


34.0 


175 


35.0 


60 


7-1 


36 


s8.o 


34 


4.3 


133 


6.3 


174 


56.0 


139 


43^o 


13 s 


140^0 


178 


104^0 


30 


6.0 


73 


l6.o 


25 


3-' 



6.4 


36.0 


6.6 


130 


4^3 


15.0 


4.9 


7-9 


3^4 


7.5 


I. 2 


3. 1 


1.8 


3^o 


33 


4.8 


2^3 


7-7 


31 


3-4 


4-5 


40.0 


1.8 


3^7 


2. 3 


4.4 


•3 


3-3 


•9 


6.4 


.4 


2.S 


3.6 


3^0 


2^7 


9.8 


1-7 


8.6 


•4 


131. 


2.g 


12.0 


.6 


2. 1 


.8 


4.0 


.3 


5-8 



336 
278 
267 

33s 

276 

119 
134 

31-' 
358 
238 
298 
179 

ao3 



89 
J30 

73 
140 
369 
303 
561 
346 

60 
108 

57 



o After U. S. Geological Survey. 



FRESH- WATER MUSSELS. II7 

DISSOLVED GASES. 

Air is inconspicuous, yet nothing is more important to man. Without it he dies; 
and his comfort, health, and normal development depend upon the purity of the air 
by which he is surrounded. This is because of the absolute necessity for oxygen, and 
the deleterious efifect of too much carbonic-acid gas. The gases dissolved in water 
are as invisible as air, but the mussels are as dependent upon the free oxygen in solution 
in the water as man is dependent upon the oxygen of the air. The water of streams 
and lakes dissolves air at the surface from the atmosphere and derives it from the 
physiological action of plants in light. Cold water will hold more free oxygen than 
warm, but the absorption of oxygen at the surface is favored by increased evaporation, 
with warm dry air and the prevalence of winds (W. E. Adeney, in Report of the Metro- 
politan Sewerage Commission of New York, 1912, p. 81). Falls, rapids, and swift 
currents promote the absorption of oxygen, and circulation currents lead to its better 
distribution into the deeper parts and throughout the whole body of water. Even 
without the aid of circulation currents, a measure of distribution of oxygen dissolved 
at the surface is effected by diffusion and "streaming" of the gas within the water 
(W. E. Adeney, loc. cit., p. 82). 

Carbon dioxide (COj), commonly called carbonic-acid gas, which is given off as a 
waste product of mussels and other animals, and which is also formed by the decom- 
position of animal and vegetable matter, is helpful in small quantities, but is poisonous 
to animals when present in too great quantities (Shelford, 1913, p. 59; 1918, pp. 39, 40; 
and 1919, p. 106). It is used up by green plants in sunlight and is also given off to the 
atmosphere at the surface of the water. The same conditions that are favorable to the 
absorption of oxygen are also favorable to the loss of COj. 

Carbon dioxide is of especial significance sometimes because of its tendency to 
unite with calcium carbonate to form the bicarbonate, which is soluble in water. Since 
the shell of a fresh-water mussel is composed principally of calcium carbonate it is liable 
to be attacked by free carbon dioxide in the water and taken up into solution. The 
horny covering of the shell is a protection against the action of the gas, but if that 
becomes broken or worn off in spots, as frequently occurs, the shell is exposed to the 
destructive effect of the acid. This leads to little harm in hard waters where the CO, 
may unite with the calcium carbonate derived from rocks or soils, but in soft waters, or 
in any waters where there is an excess of gas over dissolved calcium, the shells are 
partially or completely destroyed by corrosion. On many rivers "baldhead" shells 
are commonly encountered, and sometimes the shells are full of pits or even eaten clean 
through in the older parts. 

Nitrogen, though an important element in the composition of mussels, can not be 
used by them in the form of a gas, and its presence in water (unless in excess) is pre- 
sumably a matter of indifference to them, just as the nitrogen which composes the 
bulk of the atmosphere is uninjurious to men and not directly utilized by them (Shelford, 
1918, p. 36). Other gases found in water are ammonia, methane (CH^) and other 
hydrocarbons, and hydrogen sulphide (HjS), ^vhich are formed in certain processes 
of decomposition (Needham and Lloyd, 1916, p. 47). These are of importance only 
when occurring in sufficient quantity to be injurious. 



Il8 BULLETIN OF THE BUREAU OF FISHERIES. 

Mussels and olher animals grow more plentifully in regions of water where, with 

other conditions favorable, there is a proper gas content — abundant free oxygen and 

limited amounts of carbon dioxide. Such places are near zones of wave action in 

lakes and in rapids in streams, where the influence of green plants is felt, and where water 

circulation is good. 

VEGETATION. 

In many lakes and streams in protected locations rooted plants occur in more or 
less abundance. If this vegetation is of open character, not producing a heavy shade, it 
frequently harbors an extensive mussel fauna (Baker, 1916, pp. 94 and 95). This kind 
of habitat is especially favorable to many fishes," and to this fact in part may be attrib- 
uted the presence of mussels, since the young mussels upon leaving the fish, having 
small power of locomotion, will remain where they fall if the habitat is at all suitable. 
Since mussels are found in abundance where there is no vegetation, as in rivers like the 
Mississippi, and generally are conspicuously absent from dense growths, it would seem 
that the association with rooted plants is largely incidental. There is other direct 
evidence to indicate that mussels of such habitats are those that are parasites upon 
species of fish that have a preference for such an environment. 

vShira's observations in Lake Pepin (unpublished manuscript) indicated a certain 
association of juvenile mussels and vegetation, since 94 per cent of the juvenile mussels 
taken in a survey conducted in 191 4 were taken in situations where more or less vegeta- 
tion was encountered. On the other hand, he found juveniles at as many stations 
without vegetation as with it. As the result of many observations he concluded that a 
dense growth of vegetation was distinctly unfavorable to the survival of young mussels, 
and«he suggests that the association of juvenile mussels with vegetation may be partly 
due to the fact that environments marked by the presence of aquatic plants are attractive 
to fish. He also observed that a given area of bottom supportive of mussels might 
display a heavy growth of aquatic plants one year but be practically or entirely free 
of them in another year. The same author has observed relatively dense growths of 
vegetation on mussel beds in Lake Pokegama. 

It has frequently been observed in lakes that mussels live abundantly in patches of 
Chara, a low-growing green plant usually containing a considerable proportion of 
calcium carbonate. In the Grand River, Mich., Coker noted that mussel collecting was 
invariably poor in the midst of abundant rooted plants. The principal species found 
in such localities were the floaters {Anodonta grandis), the fat mucket (LMmpsilis luteola), 
and the pink heel-splitter {Lampsilis alata). The mucket (Lampsilis ligamentina) , and 
other species were likely to be found in the vicinity of rooted aquatic plants. 

As quoted on page no, above, Wilson and Danglade (1914, p. 15) described the 
finding of mussels beneath layers of algae and weeds in Minnesota streams. 

It must be remarked that rooted plants are not the only ones that contribute to the 
oxygen supply and to the depletion of the carbon dioxide of the water. There are 
thread algse and innumerable microscopic floating plants which play an important if 
not the most important part in oxygenation of the water, and these are widely dis- 
tributed in all zones to which sunlight penetrates. 

o "Little fishes and the greater number of mature fishes keep more or less closely to the shelter of shores and vegetation" 
(Needham and Lloyd, 1916, p. 23). 



FRESH-WATER MUSSELS. 1 19 

ANIMAL ASSOCIATES. 

In the previous discussion of the Naiades in relation to the physical environment, 
there has been shown to be an adaptation by certain species to particular physiographic 
situations, as to pond, lake, river, swift or quiet water, hard or soft bottom, etc. In 
any habitat each mussel is in association with other mussels of the same or other species 
and with animals and plants of various classes, all more or less adapted to the same 
environment. Such an association of organisms forms a community, the members of 
which interact more or less upon one another and upon their environment. The con- 
sideration of these communities with reference to th^ir members and to the environment 
often reveals important relations. Because of the mutual relations existing, a dis- 
turbance or destruction of any one element, by affecting a balanced condition, may 
cause a marked disturbance of the whole community. (See Shelford, 191 3, p. 17.) 
Some of the relations between mussels and their associates may be described as com- 
petition, symbiosis and commensalism, parasitism, and preying. A description of a 
typical habitat with its inhabitants will illustrate the variety of life associated with 
mussels. For Oneida Lake, N. Y., Baker (1916, p. 94) gives an account of a particular 
sort of habitat which he designates the bulrush-waterwillow type, where there is not 
great exposure to waves, where the bottom is more or less covered with stones and 
bowlders, but with sandy spots here and there, where the depth varies from i to 4 
feet, and where the vegetation consists of bulmshes, waterwillows, and pickerelweed. 

The principal differences between this habitat and the bowlder type are the less exposed situation, 
the density of the vegetation, the deeper water, and the sandier bottom. Such a habitat is particularly 
favorable for black bass, sunfish, rock bass, and others, because of the hiding and breeding places 
provided by the thick vegetation, the attachment for eggs by the roots and stems of plants, and the 
excellent feeding ground, by the abundance of animal life, insect, crustacean, and molluscan. The 
largest number of molluscan species, 39, occur in this type of habitat, including upwards of 15 which 
are known to be eaten by bottom-feeding fish. [The following numbers of species are listed; Mussels, 
ir, including several species of Anodonta and Lampsilis; univalves, 16; crustaceans, i (crayfish); 
Sphaeriids, 10; leeches, 5; insects, 4.] 

A typical association of mussels and other species in Andalusia Chute, Mississippi 
River, near Fairport, Iowa, is as follows (Howard, unpublished notes) : 

Bottom — gravel , rock , and sand . 

Water — depth -5':^ to 7,'^ feet. Current at surface estimated 2 miles. 

Haul — 250 feet in length, with crowfoot drag 10 feet wide and with dredge 18 inches wide. 

Distance from edge of water — 20 feet. 

Mussels — Lampsilis gracilis, 3; Plagiola elegans, i; P. donaciformis , 3; Quadrula ebenus, i; Q. 
metanevra, 3; Q. pustulosa, i; Q. undata, 2; Strophiius edentulus, i; and Unio gibbosus, 3. Total, 18. 

Bivalve — Musculium transversum Say, i. 

Bryozoa — Plumaiella polymorpha Kraepelin, i colony. 

Snail — Vivipara subpurpurea Say, 36; Plcurocera elevattim, Say i. 

Flatworm — Planarian . 

Leech — Placobdella parasitica Say. 

Insects— Stonefly, Perla sp. (larvae); mayfly, Chirotenetes, i (larva); Heptagenia, 14 (larvse); 
Polymitarcys, 2; dragonfl)', Gomphus externus, 5, Argia, 3 (larva), Neurocordulia, i; caddisfly. Hydro- 
psyche, 70 (larvae); beetle, Pamids, 2 (adult). 

Crustacea — Crayfish, Cambams. 

In communities of animals and plants, as the individuals increase in numbers there 
may develop the keen competition for food which has been designated as the struggle 



I20 BULLETIN OF THE BUREAU OK FISHERIES. 

for existence of the animate world. Since mussels feed upon suspended matter, living 
or dead, which they filter from the water, and since water once filtered must be less 
richly supplied with food for other mussels, an actual competition for food undoubtedly 
exists. Clark and Wilson (191 2, pp. 19-20) give an account of a measured area of i 
square meter (10.76 square feet) in which they counted 81 mussels and 57 other mollusks, 
making a total of 138 individuals, or about 13 per square foot; and there were present, 
of course, many other animals, some of which took the same kind of food as the mussels. 
This recorded determination of numbers per given area illustrates the possibilities of 
competition. As'indicated on pages 91 and 93, above, a detrimental competition for 
organic food probably does not occur ordinarily with mussels. 

Symbiosis and commensalism exist in such communities. A few supposed instances 
affecting mussels are afforded by small forms that live within the shells in the mantle 
cavity of the mussel where they receive food and protection. A small bristle worm, 
Chcetogaster limruEi, frequently observed in the mantle cavity of mussels, is supposed 
by some to be merely a commensal, but it may be considered a predacious species since 
it has been seen with juvenile mussels within its digestive tract (Howard, paper read at 
meeting of American Fisheries Society, 1918). The leech, Placobdella montijem, enters 
living mussels, but is not known to feed upon them (Moore, 1912, p. 89). 

Bryozoa and other sessile forms are found attached to the exposed portions of 
live-mussel shells. Doubtless there are many cases of commensalism to be revealed 
by closer study of mussels in their natural habitat. 

An interesting symbiotic relation exists between a mussel and the bitterling, a 
small European fish, which lays eggs in the mantle cavity of a fresh-water mussel which 
in turn infects the fish with glochidia (Olt, 1893). A different relation, which shows 
some reciprocity, however, is that of the fresh-water drum {Aplodinotus grunniens) 
of the Mississippi Basin, that eats fresh-water mussels but pays for the privilege, 
in part at least, by nourishing the young of several species parasitically encysted 
on its gills. (Surber, 1913, p. 105, and Howard, 1914, pp. 37 and 40.) The same is 
true of other fish that eat mussels, as the catfishes. 

Parasitism is a phenomenon of community relations, and it is of double significance 
in the case of mussels, because not only have the mussels parasites to prey upon them, 
but they with few exceptions depend for existence upon the opportunity to become 
parasites of fish or, in one case, of an amphibian. A rather close relationship of fish 
to the mussel community is essential, and there may be a particular interrelation of 
given species of fish and of mussels. Questions arise as to when and how this special 
and intimate relationship came about and to what extent the habits of host and 
mussel interlock in such cases as the gar pikes and the sand-shells (Hovt^ard, 1914a), 
the river herring and the niggerhead, the shovel-nose sturgeon and the hickory-nut, 
the catfiches and the warty-back, the mud puppy and the little salamander mussel. 
In the last-named case, the peculiar habit of the mussel which lives beneath flat stones 
conforms evidently to the habits of the host, for the mud puppy is well known to 
frequent such situations. 

One feature of certain mussels that possibly serves to decoy fish is the elaborate 
development of the mantle flap in gravid females of the pocketbook mussel, Lampsilis 
ventricosa, and others. (See p. 85.) These flaps in their form and coloration, includ- 
ing an eyespot, resemble a small fish, and the motion of these in the current still further 



FRESH- WATER MUSSELS. 121 

enhances the resemblance. The enlarged marsupia distended with glochidia lie close 
to these flaps, one on each side. It has been suggested that a fish darting at this 
tempting bait may cause the extrusion of the glochidia and then become infected. 
(See Wilson and Clark, 1912, pp. 13, 14.) The unwelcome members in the associations 
to which mussels belong are discussed in the following section on "Parasites and 
Enemies." 

PARASITES AND ENEMIES. 

PARASITES. 

Long green algae are occasionally found attached to the exposed tips of the shells 
of mussels, and these may cause some erosion of the shells. Marly concretions, com- 
posed of intermingled low algae and lime often form knoblike lumps on shells in lakes. 

Among the most common of mussel parasites are water mites which dwell in the 
gill cavity and lay their eggs within the flesh of the mussel, in the inner surface of the 
mantle, in the foot, or in the gills. These water mites, which belong to the genus Atax, 
vary in size and color and to some extent in shape (Wolcott, 1899). One is black with 
a white Y-like marking on its back; others may be reddish. The largest and most 
degenerate is of a honey color with white treelike markings, but because of its incon- 
spicuous coloration it is often overlooked. The different sjjecies of Ata.x are hard to 
distinguish without special preparation and study. Under magnification these water 
mites look somewhat like spiders. Small pearls are sometimes formed about Atax eggs. 

Leeches are occasionally seen on the inner surface of the mantle of some mussels, 
especially in Anodontas (floaters) in ponds. They probably feed on the mucus of the 
mussel. 

A small organism closely resembling a minute leech in general shape and appear- 
ance is occasional in the axils of the gills of mussels in some lakes. This is Cotylaspis 
insignis, one of the trematodes or flukes (Leidy, 1904, p. no). One mussel may 
harbor a dozen or more of these parasites. Rather similar to Cotylaspis insignis but 
considerably larger and pink instead of yellowish, is the trematode Aspidogaster con- 
chicola. It is more complex than Cotylaspis insignis and is usually found in the peri- 
cardial cavity of the host mussels, although in severe infection it may overflow into 
other organs. 

Distomids, both free and encysted, are found in mussels. The distomid occurs 
in almost any muscular part of the body but most frequently in the foot or along the 
edges of the mantle. Sometimes pearls are fonned around distomid cysts. The free 
distomids are usually found on the mantle surface next to the shell; they are chiefly 
confined to the flesh along the hinge line but may extend lower down. They are often 
associated with small irregular pearls. Sporocysts of distomids are common, especially 
in some Quadrulas. Many distomid parasites of mussels appear to be harmless, but 
one, Bucephalus polymorphus, destroys their reproductive organs (Kelly, 1899, p. 407; 
Wilson and Clark, 1912, pp. 69, 70; Lefevre and Curtis, 1912, p. 121). An ascarid 
worm is occasionally found in the intestine of mussels. 

A worm with peculiar hooks on its head was found encysted in the margin of the 
mantle of some mussels in a pond near Fairport, Iowa. It was probably a trematode 
but has been found only once and never identified. 



122 BULLETIN OF THE BUREAU OF FISHERIES. 

An oligochaete worm, Chcetogaster limtUBt, is occasionally found in mussels. It is 
possibly a parasite of snails from which it now and then migrates to mussels. We have 
some reason to believe that it devours the other mussel parasites. The crystalline 
style, a long translucent gelatinous body which is formed by the mussel within its in- 
testine, is often mistaken by clammers for a womi. 

Certain protozoa, Conchopthirus curtus and Canchopthirui anodonUz, somewhat 
resembling in general appearance the slipper animalcule, Paramoecium, are occasionally 
met in the mucus of mussels. Attached protozoa, like \^orticella, are also occasionally 
found on the edge of the mantle. 

Occasionally larval Atax migrate into the space between the mantle and shell 
and are covered by nacre, where they may form minute white tracks, or in some cases 
apparently small raised "blisters" or pimples (Clark and Gillette, 191 1). One or 
perhaps several species of distomid causes a brick-red or purplish discoloration of the 
nacre, mostly in thin-shelled mussels (Anodonta and Strophitus) (Osbom, 1898; Kelly, 
1899, p. 406; Wilson and Clark, 191 2, p. 66). The marginal cyst distomid sometimes 
causes a steel-blue stain of the nacre near the margin (Wilson and Clark, 1912, p. 63). 

ENEMIES. 

Mussels have numerous enemies, among which may be mentioned the mink, the 
muskrat, the raccoon, water birds, turtles, fishes, hogs, and man. 

Of the depredation of many of these we know little. Water birds probably kill 
but few mussels, and of fishes, catfish and the sheepshead, or fresh-water drum, are 
the most noteworthy. These probably feed mainly on the thinner-shelled species. 
Small mussels {Lampsilis parva) have been found in the intestines of the turtle, Mala- 
clemmys lesueurii. 

Besides man the muskrat is the most notorious enemy of mussels, and the shell 
piles left by them are often conspicuous objects along the shores of lakes and rivers. 
Conchologists sometimes rely upon the muskrat's shell piles to furnish them choice 
and rare shells. Evermann and Clark (1918, p. 284) found not a few examples of 
Micromya fabalis in muskrat shell piles on the banks of Lake Maxinkuckee, though 
collecting in the lake during several seasons failed to reveal a single living specimen. 
Clammers prospecting new rivers sometimes use the piles of shells left by the muskrat 
as aids indicating where to dredge for shells. 

Direct observations of the work of muskrats in Lake Maxinkuckee, Ind., were made 
by Clark and reported in "The Unionidse of Lake Maxinkuckee" (Evermann and Clark, 
1918, pp. 261, 262), as follows: 

The greatest enemy of the lake mussels is the muskrat, and its depredations are for the most part 
confined to the mussels near shore. The muskrat does not usually begin its mussel diet until rather 
late in autumn, when much of the succulent vegetation upon which it feeds has been cut down by 
the frost. Some autumns, however, they begin much earlier than others; a scarcity of vegetation or 
an abundance of old muskrats may have much to do with this. The rodent usually chooses for its 
feeding grounds some object projecting out above the water, such as a pier or the top of a fallen tree. 
Near or under such objects one occasionally finds large piles of shells. The muskrat apparently has 
no especial preference for one species of mussel above another but naturally subsists most freely on 
the most abimdant species. These shell piles are excellent places to search for the rarer shells of the 
lake. 

In the winter after the lake is frozen, great cracks in the ice extend out from shore in various 
directions, and this enables the muskrat to extend his depredations some distance from shore in defi- 



FRESH- WATER MUSSELS. 123 

nite limited directions. During the winter of 1904 a muskrat was observed feeding on mussels along 
the broad ice crack that extended from the end of Long Point northeastward across the lake. The 
muskrat was about 50 feet from the shore. It repeatedly dived from the edge of the ice crack and 
reappeared with a mussel in its mouth. Upon reaching the surface with its catch it sat down on its 
haunches on the edge of the crack and, holding the mussel in its front feet, pried the valves apart 
with its teeth and scooped or licked out the contents of the shell. Some of the larger mussels were 
too strong for it to open, and a part of these were left lying on the ice. The bottom of the lake near 
Long Point, and also over by Norris's, is well paved by shells that have been killed by muskrats. 
Muskrats do not seem to relish the gills of gravid mussels; these parts are occasionally found untouched 
where the animal had been feeding. 

In spite of all these enemies mussels held their own and throve and flourished 
imtil the appearance of man upon the scene, when depletion of the mussel beds became 
noticeable. Man exterminates a good many mussel beds by sewage discharge, by 
drainage, through which sand is washed do%vn over the beds, by dredging and construc- 
tion of %ving dams for navigation, by pearling, but, most of all, by exhaustive clamming 
for the shells. 

CONDITIONS UNFAVORABLE FOR MUSSELS. 

Since mussels are animals of generally sedentary habit, with limited powers of loco- 
motion, they arc more helpless to escape from unfavorable conditions of environment 
than are fish or other active creatures of the water. This relative helplessness does not 
characterize the adult mussel alone, but is even exaggerated for the young stages. 
From the time the larval mussel attaches itself to a fish until it is liberated it is entirely 
dependent upon the movements of its host for its future home ; it may be dropped in a suit- 
able environment or in a place wholly unfavorable to its survival. On the other hand, 
adult mussels of many species can endure unfavorable conditions for a considerable 
period of time. This is found to be especially true of several species of Quadrula. 

NATURAL CONDITIONS. 

Some natural conditions unfavorable to mussel life are shifting bottom, turbidity, 
sedimentation, floods, and droughts. These conditions pertain usually to streams 
rather than to lakes. They have received some consideration in various paragraphs 
of this section on "Habitat"; therefore it is only necessary to summarize them in 
this connection. 

The paucity of mussels in the Missouri River, as well as in the greater part of the 
Red River and other streams of the plains, is no doubt due to its exceedingly shifting 
bottom. Similar conditions apply in lesser degree in the lower stretches of many 
streams. In fact, all rivers, for some distances above their mouths, are as a rule very 
deficient in mussels as compared with sections farther up where bottom and other con- 
ditions are more favorable. Shifting bottoms not only prevent mussels from securing a 
foothold, but may also entirely destroy established beds. 

Interrelated with shifting bottom are turbidity and sedimentation. All three factors 
and the extent to which they may be operative are largely dependent upon flood condi- 
tions. In nearly all large rivers floods commonly plow new channels here and there in 
the stream bed, cut away banks to a greater or less extent, and build new shoals or 
change the form and dimensions of old ones. Such changes in navigable streams are 
9745°— 21 i 



124 BULLETIN OF THE BUREAU OF FISHERIES. 

familiar to pilots who find it necessary to ' learn the nver " each season. Many of these 
changes must be catastrophic to mussels in certain localities. 

Excessive turbidity with consequent increased sedimentation, when of considerable 
duration, is no doubt seriously unfavorable to the well-being of mussels. It has been 
stated that mussels do not feed during periods of high turbidity, but no definite data 
in support of this can be given. That mussels do not "bite" well on the crowfoot 
hooks during a rising stage of water is a condition recognized by clammers. Whether 
the fact that the shells are not generally open and the mussels feeding at this time is due 
to the turbidity, or to other changing conditions incidental to the rising water, can not 
be stated. If heavy deposits of sediment are unfavorable for adult mussels, they must 
be more directly harmful to the young during the early stages of independent life, for 
the tiny juveniles may be smothered by deposits that would have less disastrous effect 
upon larger mussels. 

The effects of droughts are ordinarily felt but little by the mussels of the larger 
streams and lakes. The most unfavorable condition arises when, owing to a prolonged 
dry season, the water is lowered to such an extent that the mussels fall easy prey both 
to muskrats and to clammers and pearlers seeking them in the shallow water. Crows, 
too, are known to pluck out and kill Anodontas when the water over them becomes low 
and clear. 

Inthesmallstreams, lakes, and sloughs, the mussels may be killed by the partial or 
complete drying up of the water. Certain species of mussels are, of course, more resis- 
tant to such condition than others. Isley (191 4) states that live specimens of Unto 
tetralasmus were plowed up in a pond three months after it had become dry. The mus- 
sels had burrowed down to zones of moisture. 

ARTIFICIAL CONDITIONS. 

Among the conditions imposed by man that may be detrimental to mussel life in 
our streams may be mentioned the discharge of sewage, industrial wastes, dredging, 
and the building of wing dams. (See Pis. IX, X, and XI.) 

Disposition of the sev^age and wastes of large cities without harmful contamination 
of the rivers presents an issue of growing importance. Portions of streams just below 
important cities are sometimes veritable cesspools, unsuited to both mussel and fish life. 
The Illinois River for a considerable distance below its origin is greatly influenced by 
sewage pollution through the Des Plaines River and the drainage canal ; from the head 
of the stream down to Starved Rock, 42 miles from the source, no mussels are found, 
and a normal variety and abundance of fishes is not present above Henry, 77 miles from 
its source (Forbes, 1913, p. 170; Forbes and Richardson, 1919, p. 148). Industrial 
wastes from pulp and paper mills, tanneries, gas plants, etc., are injurious to fishes, 
and no doubt harmful to mussels as well. Such unfavorable conditions as arise through 
the depletion of oxygen supply by the decomposition of sewage are partially or com- 
pletely corrected by the intervention of rapids or waterfalls. (See Shelford, 191 9, 
p. Ill, and Baker, 1920.) 

River improvement work, such as dredging and the building of wing dams, creates 
conditions more or less unfavorable for mussels. Hydrauhc dredging may destroy 
mussels, either directly by pumping them up, or by shifting the river channel so that 



FRESH- WATER MUSSELS. 1 25 

ensuing changes cause new sand bars to form and to bury previously existing beds. 
Wing dams constructed for improvement of the Mississippi River, built of rock and 
brush and projecting from the shore to the channel, have far-reaching effects upon the 
course of the current, upon sedimentation, and upon the formation of sand bars. The 
area between the dams may fill up with sand, so that eventually willows are growing 
where a mussel bed once flourished. Such changes have been observed in the Mississippi 
River near Fairport, Iowa, and at Homer, Minn. 

The effect of the construction of dams directly across the channel of a river, as for 
water-power development, has been discussed on page 97. 

Greater irregularity of stream flow resulting from the clearing of forests greatly 
influences the life of mussels. The drying up of ponds inhabited by mussels and the 
extreme low stages of water which allow clammers to obtain the mussels by wading, form 
disastrous conditions to which mussel beds are occasionally exposed. Extreme low 
stages of lakes and streams in summer may lead to mortality of mussels resulting from 
high temperature of the water and diminished oxygen supply. (See Strode, 1891; 
Sterki, 1892; Farrar, 1892.) 

GROWTH AND FORMATION OF SHELL. 

MEASUREMENTS OF GROWTH. 

Methods of propagation, estimate of results, and measures for protection all depend 
in a considerable degree upon knowledge of the rate of growth of mussels. It is impor- 
tant to know how many years elapse before a mussel may attain a market size, as well 
as at what age it may be expected to begin breeding. Furthermore, these questions 
require answers for more than 40 economic species, even if consideration were not given 
to the more than 500 additional American species of fresh-water mussel. The rate of 
growth is not, however, easily ascertainable for most species. 

Mussels of any species may be left under observation for a considerable period in 
tanks or troughs, but experiments indicate that normal growth does not occur under 
the conditions of life in tanks. Even large ponds do not offer the conditions required 
by many species. The data to be offered on this subject are derived principally from 
experiments conducted at the Fairport station. Further data on growth of mussels 
will be found in Isley's paper (191 4). 

Pocketbooks, Lampsilis ventricosa, reared in one of the ponds at the Fairport 
station attained a length of 41 to 47 mm. (1.6 to 1.85 inches) in two growing seasons, 
and about 65 mm. (2.56 inches) by August of the third season. Examples 45 to 47 
mm. long (1.76 to 1.85 inches), and these evidently in the second year of free life, were 
measured and planted in the Mississippi River by Lefevre and Curtis in June, 1908, 
and recovered by the senior author of this paper in November, 1910, at the close of the 
fourth year of growth (Lefevre and Curtis, 1912, p. 180 ff). They had attained lengths 
of 81 to 85 mm. (3.18 to 3.35 inches). (See fig. 6, p. 133.) 

It is evident, then, that pocketbook mussels under only ordinarily favorable con- 
ditions may attain a marketable size by the end of the fourth season of independent 
life (at 3K years of age from date of infection). The observations reported in the 
following table (10) show that a nearly equal rate of growth applies to the Lake Pepin 
mucket, Lampsilis luteola. 



126 



BULLETIN OF THE BUREAU OF FISHERIES. 



Table io. — Average Length of Six Examples of the Lake Pepin Mucket, Lampsilis luteola, 

Reared in Pond 3D at Fairport, Iowa. 



Time of measurement. 


Length. 






MillinuteTS. 

43-4 
68.8 
77.0 
80.6 
84.9 


Inches. 

I. 71 
2.73 
3.04 
3.18 
3-35 


Close of third growing; season o 











1 Tlie records of the original lot for the third year having been lost in the fire, there is substituted a corresponding record for 
the third year of mussels of another lot recorded in Pond 8D. The mussels in Pond 3D were from a fall infection and those in 
8D from a spring infection; therefore the former are sUghtly older. 

Another species of pocketbook, Lampsilis (Propiera) capax, had attained a length 
of 49 mm. (1.93 inches) at the end of the second season, indicating a slightly more 
rapid growth for this species than for Lampsilis ventricosa. Thinner-shelled species of 
the genus Lampsilis may grow more rapidly. Thus some examples of the paper-shell 
Lampsilis (Propiera) Imvissima, known to be not over i6 months of age (in free life), 
had attained lengths of 78 to 81 mm. (over 3 inches). An example of the paper-shell, 
Lampsilis (Paraptera) gracilis, grew from 17.6 mm. (0.7 inch) to 107 mm. (4.2 inches) 
in 2 years 9 months and 18 days, the rate of growth averaging about i}4 inches per 
year. 

The very thin-shelled mussels of the genus Anodonta grow even more rapidly. 
Examples of the floater or slop-bucket, Anodonta corpulenia, taken from a pond at the 
Fairport station 16 months after the ponds were constructed, varied in length from 
66 to 88 mm. (2.59 to 3.46 inches). Examples of another paper-shell, Anodonta sub- 
orhiculata, taken at the same time from another pond of the same age, but which may have 
offered less favorable conditions, were 64 to 67 mm. in length (2.52 to 2.63 inches). 

With regard to heavy-shelled mussels, such as the niggerhead, pimple-back, and 
blue-point, there is much less satisfactory evidence as to growth. They undoubtedly grow 
much more slowly than mussels possessing thin shells, yet the rates of growth secured 
in such experiments as have been conducted can hardly be assumed to be representative 
of the conditions prevailing in nature. The species are not \vell adapted to life in tanks 
or ponds, and there are few places where measured specimens can be placed in rivers 
with any assurance that they will remain undisturbed or may be recovered at a later 
time. In Lefevre and Curtis's experiments (1912) an example of the hickory-nut, 
Obovaria ellipsis, that was practically full-grown when first measured, gained 5 mm. (one- 
fifth of an inch, 0.197) i^ ^ little less than 2K years. In the same period an example 
of Quadrula solida, somewhat less mature, gained 10 ram. (two-fifths of an inch, 0.394). 

In the following table (11) there are indicated sizes, at the close of the second year, 
of certain mussels reared accidentally or intentionally in ponds at the Fairport station. 
The short-term breeders, at least, were a little less than iK years of age. 

Since these are all mussels of river habit, it can not be assumed that the growth 
attained in ponds is representative of the rate of growth in a natural environment. 



FRESH-WATER MUSSELS. 



127 



Table h. — Sizes at Close of Second Year op Certain Mussels Reared in Ponds, 

Station, Iowa. 



Fairport 



Sdentific name. 



Common name. 



lycngth. 



Lampsilis Hgamentina. . . 

Lampsilis anodontoides . 

Obliquaria reflexa , 

Obovaria ellipsis 

Plagiola donaciformis . . . , 

Quadrula plicata , 

Quadrula pustulosa 

Quadrula undata 



Mucket a 

Yellow sand-shell b 

Three-homed warty-back. 
Hickory-nut 



Blue- point. . . , jiiiU. 

Pimple-back. . .' 

Pig-toe 



Millimeters. 


Inches. 


20.0 





41 
16 






z 


II 

20 


4 





»3 

24 
15 


S 
a 
8 





.79 
.62 
•63 

•4S 
-79 
•S3 
•69 
.63 



a Other observations indicate tiiat the mucket grows more rapidly in streams, 
b The yellow sand-shell was only i year and 3 months of age. 

Some medium-sized examples of several species of Quadrula were placed, after 
measurement, in a crate which was anchored in the Mississippi River at Fairport, Sep- 
tember 19, 1910. When the crate was recovered and the mussels remeasured, July 31 of 
the following year, very little growth was apparent in most of the specimens. The data 
for measurements of length in the several examples are given in the following table (12) : 

Table 12. — Incrbase in Length of Mussei3 in Cage. 



Scientific name. 



Common name. 



Length, 


Length, 


Sept. 19. 


July 31, 


1910. 


1911. 


Inches. 


Inches. 


1.92 


1.98 


1.74 


..85 


I. 70 


1.86 


2.80 


3- 02 


..4. 


I. 74 



Increase in 
length. 



Quadrula cbenus 

Quadrula pustulosa, . 
Quadrula metanevra. 
Quadrula plicata . . . . . 
Quadrula undata 



Nigger head... 
Pimple-back. 
Monkey-face. 
Blue-point. . . 

Pig-toe 



Inches. 

o. 06 



In another experiment 76 mussels, representing 19 species, principally the thick- 
shelled forms, were placed in a crate with nine compartments which was anchored in the 
river about 25 feet from shore. The crate was put out July 31, 191 1, and recovered No- 
vember 14, 1913, when 36 of the original mussels, representing 13 species, were found to 
be alive. These mussels generally manifested a higher rate of growth than marked some 
of the mussels used in the experiment just described, although the increase in size was 
disappointingly small. The period of time between the dates of measurements was 2 
years 3 months and 14 days. The mussels were of medium size at the beginning of the 
experiment, so that the growth to be expected was that which would characterize the 
period of approaching maturity rather than that of early life. The mussels living at 
the close of the experiment^ with the maximum and minimum gain in length and the 
average for the species (when more than two examples were available), are shown in 
the following table (13) : 

Table 13. — Growth of 36 Mussels in Crate from July 31, 1911, to Nov. 14, 1913. 



Scientific name. 



Quadrula ebenus 

Quadrula pustulosa 

Quadrula pustulata 

Quadrula metanevra. . . . 

Quadrula plicata 

Quadrula undata 

Obovaria ellipsis 

Obliquaria reflexa 

Tritogonia tuberculata. , 
Lampsilis ligamentina. . 

Lampsilis recta 

Strophitus edentulus. . . , 
Unio gibbosus 



Common name. 



Niggerhead 

Pimple-back 

....do 

Monkey-face 

Blue- point 

Pig-toe 

Hickory-nut 

Three-horned warty-back. 

Buckhorn 

Mucket 

Black sand-shell 

Squaw-foot 

Spike ; 



Examples. 



Number. 
7 
5 



Increase in length. 



Maximimi. 


Minimum. 


Average. 


Inches. 
0.64 
•so 


Inches. 

0.38 
. 20 


Inches. 
0.463 
■375 


.46 


.20 




.80 


•S17 


.72 
.18 


• 4SS 


.36 






.84 


.10 




1.76 




.66 


•S6 




•17 





128 



BULLETIN OF THE BUREAU OF FISHERIES. 



It must be borne in mind that the conditions of life for mussels in an inclosed crate, 
and relatively closely crowded together, are probably not nearly so favorable for growth 
for the majority of mussels as are those of the natural river bottom, where the mussel 
has a fair chance to assume its desired position and secures the full benefit of the food- 
laden current. Doubtless the maximum rate of growth shown in the crate is more 
nearly normal than the average rate. Our impression is that thick -shelled mussels, such 
as the niggerheads, pig-toes, and pimple-backs, after they are half grown, increase 
in size ordinarily at the rate of a quarter of an inch a year or less. If this be true, it 
would require four years or more for a niggerhead mussel, under ordinarily favorable 
conditions, to increase from a length of 2 inches to a length of 3 inches. Assuming that 
the rate of growth is more rapid in early life, it may be inferred that niggerheads or 
pimple-backs 3 inches in length are 10 or 12 years of age. Additional experiments 
conducted under proper conditions are clearly wanted. 

A marked contrast in rate of growth is thus afforded by the species of Quadrula (and 
others having generally' similar character of shell), on the one hand, and those of Lamp- 
sUis, on the other. This was strikingly shown, in connection with the last experiment 
described, by two examples of the yellow sand-shell, Lampsilis anodontoides , which were 
not put into the crate but which must have found their way in by chance v/hen still 
small enough to pass through the screen wire of yi-mch. mesh. Although the crate was 
out only a little over two years, these two sand-shells were respectively 3.30 and 4.12 
inches in length. Being elongate in form, they may have entered the crate when a 
little more than an inch in length. 

Table 14 embodies the result of measurements of length and counts of rings on 
yellow sand-shells, Lampsilis anodontoides , from the St. Francis River, at Madison, Ark. 



Table 14. 



-Classification of 40 Yellow Sand-Shells from St. Francis River, Ark., 
TO Length and Age. 



According 



Length in 


Num- 
ber 
each 
leugth. 


Age as indicated by interruption rings on 
surface of sheU. 


Length in 
inches. 


Num- 
ber 
each 
length. 


Age as indicated by interruption rings on 
surface of shell. 


inches. 


Three 

years. 


V Four 
years. 


Five 

years. 


Six 
years. 


Older. 


Three 
years. 


Four 
years. 


Five 
years. 


Six 
years. 


Older. 




I 
I 
z 
2 
4 
S 


I 

2 

I 


oi 








4K 

4^ 

aH 


6 
3 
9 

S 




s 
i 

2 


I 






3^.";::::::: 












3K 


2 
2 
6 








S 

t 


I 
2 




3J4 










ti/ 








Total... 










I 






40 


4 


22 


8 


3 













" shell with stunted appearance. 



The observations indicate that mussels of this species in the St. Francis River attain 
a length of 4 to 4K inches in 4 years, that they may attain a length of 4 inches in 3 years, 
and that 6 years or more are ordinarily required to attaiin a length of 5 inches. 

In summary, the rate of increase in length of fresh-water mussels varies from i K or 2 
inches per year for paper-shells (as Lampsilis IcBvissima) to J4 inch (a little more or a little 
less) per year for the niggerhead and related species, while an intermediate rate of J^^ or i 
inch per year characterizes the muckets and pocketbooks, and a slightly more rapid rate 
the sand-shells. In general the rate of growth is so directly proportioned (in inverse 



FRESH-WATER MUSSELS. 129 

ratio) to the thickness of shell of the species as strongl}' to suggest that the limiting 
factor of growlh ordinarily is not organic food, but the mineral content of the water 

(p. 87). 

PRESENCE OF SO-CALLED GROWTH RINGS. 

The ages of animals may not infrequently be determined, at least approximately, by 
the "rings of growth," on teeth, scales, scutes, or otoliths (ear stones), or other hard parts 
of the body. A similar criterion of age determination is of course commonly applied to 
trees. More recently the rings on the scutes of terrapin and those on the scales and 
otoliths of fish have been used for the same purpose. 

This method of determining age is generally based upon the belief that the cessation 
or the slowing down of growth during the winter season may cause the formation of a 
distinguishable line or band on a concentrically growing structure. By counting the 
number of winter lines or bands the number of winters through which the animal has 
passed is ascertained, or by counting the number of zones between such rings, beginning 
with the center zone, the number of seasons of growth is discovered. It is one thing to 
know that such rings are formed in winter, but quite another thing to learn just how or 
why the rings are formed. It is also of primary importance to determine whether or not 
similar rings may be formed upon any other occasion than the occurrence of a season of 
winter. In the case of the fresh-water mussel shell, at least, these questions can be 
answered by observations and experiments. (Coker, unpublished notes.) 

Some years ago when collecting mussels in lakes in southern Michigan it was ob- 
served that the shells of the fat muckets were all marked with several conspicuous rings 
which were approximately equally spaced on all the mussels of a bed. It seemed a 
natural inference that these dark rings represented winter periods and thus afforded a 
means of age determination. At another time, upon examination of mussels which 
had been measured and placed in crates in the river two years previously, it wasobsei^ved 
that there were rings apparently corresponding to the two winters which had elapsed 
since the date of original measurement, but that there was also another ring which 
marked the exact size of the mussel when originally measured. (See text fig. 6.) 
Subsequent observations showed that whenever a mussel was measured and replaced in 
the water, a ring would be formed on the shell before growth in size was resumed. 

These obser^^ations led to an effort by microscopic examination of sections of the 
shell to determine the significance of rings which apparently could be formed either by a 
season of cold weather or by the pi ocedure of taking a mussel from the water, applying a 
caliper rule, and returning it to the water. To make clear what was learned from the 
study of the sections it is necessary first to explain briefly the mode of formation of shell 
which leads to gro\\i;h in size. 

MODE OF FORMATION OF SHELL. 

The shell is composed of four distinct layers (text figs. 1,2, and 3). The outer is the 
homy covering called the periostracum." Immediately beneath this is a calcareous 
layer composed of prisms of calcium carbonate set vertically to the surface. This pris- 
matic layer is very thin, though thicker than the periostracum, and is likely to remain 

•* The fact that the periostracum itself comprises 3 layers of separate origin, while very siguificailt in some respects, is imma- 
terial in this comiectiou. 



130 



BUI^LETIN OK THE BUREAU OK FISHERIES. 




Fio. I. — ^Biagraimnatic and highly magnified camera lucida drawing of section of 
margin of fresh-water mussel shell, Obovaria ellipsis (Lea), showing arrangement of 
layers: A^ epidermis (double layer); B, prismatic layer; and C, nacreous layer. 
Note the folds of epidermis which give the shell its "silky" appearance. 



attaxihed to the periostracuni when that is peeled off. Beneath the prismatic layer and 
composing nearly the entire body of the shell is the nacreous or mother-of-pearl layer, 

w hich is made upof almost 
innumerable thin laminae 
lying one upon the other 
and parallel to the inner 
surface of the shell. 
Through the nacre, inter- 
secting its laminae, passes a 
very thin fourth layer, the 
hypostracum," secreted 
by the muscles (p. 172). 

Growth of shell in 
thickness is accomplish- 
ed by the laying down of 
successive laminae, from 
the entire surface of the 
mantle. Layer after layer is added to the inner surface of the shell, each layer exceed- 
ingly thin and generally a little larger than the preceding. Ring after ring is added 
to the margin of the shell, but since growth is most 
pronounced in the posterior (rear) direction, less 
so in a ventral, and still less in the anterior 
(forward) direction the rings must be widest be- 
hind and narrowest in front. It will be noted 
that any mussel shell is marked with innumerable 
concentric lines. Superficially such lines suggest 
the annual rings seen on the section of the trunk 
of a tree, but the resemblance is entirely mislead- 
ing. The shell is added to in layers, but a very 
great number of layers are made in a year. 
Pfund (191 7) has, by refined physical methods, 
measured the thickness of the layers or laminae 
and determined that the thickness in the examples 
he studied lies between 0.4 /i and 0.6 /i. Transla- 
ting these terms into ordinary language, there are 
some 50,000 layers to an inch of thickness. A shell 
one-quarter of an inch thick would have 1 2,500 lam- 
inae; and if such a shell were 8 years old, more than 
1,500 laminae would have been formed each year, 
on the average. The outcropping edges of these 
laminae on the surface of a polished niggerhead shell 
have also been measured and found to be spaced at 
the rate of about 9,000 to the inch. Such lines are of course not visible to the naked eye, 
and therefore the fine rings in evidence on the surface of the shell can not represent these 





Fig. 2. — Section through double-layered pert- 
ostracum and prismatic layer. Nacreous layer 
below not shown. 



a Not shown in figures herewith. 



FRESH-WATER MUSSELS. 131 

laminae but must have some other significance. They probably mean nothing more 
than slight and frequent but irregular retractions of the margin of the mantle during 
the process of shell formation, which have registered themselves in fine wrinkles on 
the surface of the shell as it is built. The more conspicuous rings that mark some 
shells still await our attention. 




Fig. 3. — Sections through prismatic layer of Quadrula ehenus. The sections were made at different levels, the prisms being smaller 

and more numerous in the outer portion. X 300. 

Growth of the shell in length and breadth is accomphshed by the secretion of shell 
substance of the three layers by cells at or near the margin of the mantle. There are 
certain cells of a furrow in the margin of the mantle which form only periostracum, 
and there is a certain portion of the mantle near the margin which forms only prismatic 
shell substance, while the greater portion of the mantle surface normally forms only 
nacre. Now, the important point for our present consideration is this: If, from any 
cause, the margin of the mantle is made to withdraw within the shell to such an extent 
as to break its continuity with the thin and flexible margin of the shell, then, as the 
study of sections indicates, when the deposition of shell is resumed, the new layers 




Fig. 4. — Section through the interruption ring on pocketbook mussel, caused by handling mussel 
in summer. Simple duplication. 

of prismatic substance and periostracum are not continuous with the old, end to end, 
but are more or less overlapped by the old. In other words, growth does not begin 
again exactly where it left off, but a little distance back therefrom, and the cause of this 
is largely mechanical (text fig. 4). The amount of overlapping probably depends 
upon the degree of disturbance and the extent to which the mantle has withdrawn 
itself. The result is an unwonted duplication of layers. Counting inward from the 



132 



BULLETIN OF THE BUREAU OF FISHERIES. 




outer surface we find not simply one series of periostracum, prismatic, and nacreous 
layers, but periostracum and prismatic layers, then periostracum and prismatic again, 
and finally the nacreous layer; the outer layers arc doubled up. 

SIGNIFICANCE OF RINGS. 

In a case such as has just been described, where the outer layers are doubled up 
as a result of an extreme retraction of the mantle, the effect of seeing a second horny 
layer through the outer periostracum and the fairly translucent prismatic layer gives 
the appearance of a dark band on the shell. This is the so-called growth ring, which 
would be better termed duplication ring or interruption ring," since its significance is 
simply that the continuity of the outer layers is interrupted and the break is repaired 
by overlapping. In other words, the periostracum and prismatic layers are "spliced" 
at this point. A duplication of layers should easily be observable on shells having 
fairly Hght-colored or translucent periostracum but not on shells having a very dark 
or opaque covering, and this is found to be the case. Growth rings or interruption 
rings are commonly seen on pocketbooks, fat muckets, yellow sand-shells, floaters, and 
other shells of light or only medium dark colors, while they are distinguishable with diffi- 
culty, if at all, on niggerheads, 
pimple-backs, blue-points, and 
other dark-colored shells. 

If the winter rings are 
formed in the same way, and 
the breaking of the continuity 
of the outer layers is due to 
the withdrawal of the mantle 
in cold weather, then it would 
be expected that several duplications would occur for a single winter. For cold 
weather does not ordinarily fall with one blow. Periods of cold and warm weather 
alternate for a time before winter sets fairly in, and again in the spring periods of low 
and high temperature alternate before winter is entirely passed. Such fluctuations 
of temperature are, of course, not so frequent or noticeable in the water as in the air, 
but they do occur. It might be expected that the mussel would react to the first sharp 
touch of winter by closure and a sharp withdrawal of the mantle but that the deposition 
of shell would be resumed after a time, while further interruptions and resumptions 
of growth would occur before the full effect of winter was experienced. Again in the 
spring there might be alternate interruptions and resumptions of growth. This, at 
least, is the story which seems to be told by a section through a winter ring when 
examined under the microscope. Text figure 5 shows such a section, where the alterna- 
tion of periostracum and prismatic layers is repeated seven times, indicating six inter- 
ruptions of growth. As virtually no increase in size occurs between the several inter- 
ruptions, the duplicated or repeated layers are simply piled upon one another. 

Interruption rings corresponding to seasons of winter differ from those corresponding 
to a single severe disturbance of the mussel during the normal period of growth in that 
the latter are rings of single duplication (text fig. 4), while the former show several repe- 
titions (text fig. 5). The winter rings in shells that have been observed are, therefore, 
darker, though they may or may not be broader (text fig. 6). 

o See Isely. 19x4, p. 18. 



Fig. s. — Section through interruption ring (winter ring) on shell of pocket- 
book, Lampsilis ventricosa, showing repeated duplications of periostracum 
and prismatic layers. 



FRESH-WATER MUSSELS. 



133 



ABNORMALITIES IN GROWTH OF SHELL. 

Seriously malformed mussels are not infrequently found, and peculiar interest 
attaches to these because shellers generally entertain the belief that a mussel with de- 
formed shell is most likely to contain a pearl. It seems possible that this belief is not 
without some foundation. Pearls probably occur more frequently in parasitized mus- 
sels, and many of the observed malformations are undoubtedly due to parasites. 

A few distomids upon the mantle of Anodontas along or near the dorsal fold evidently 
cause rusty stains in the nacre, abnonnal growths on the inner surface of the shell, de- 
formities of the hinge teeth, and dark or poorly formed pearls. Another parasite which 
infests the reproductive or- 
gans may almost completely 
destroy the gonads of the fe- 
male mussel, and in such case 
the female may develop a 
shell in the form of a male or 
in a form intermediate be- 
tween that of the male and 
the female. There is evi- 
dence that parasites found 
encysted in the margin of the 
mantle may give rise to stains 
on the nacre at the margin of 
the shell, that others cause 
the not unfamiliar steely or 
leaden-colored margins of 
shells, while some produce a 
pitting of the inner surface 
of the shell. 

One of the most common 
and serious defects of other- 
wise valuable commercial 
shells is the presence of yel- 
low and brown spots or bluish or greenish splotches in the nacre. Regardless of the 
texture of the shell, the partially or wholly discolored buttons must be given a very low 
grade. The spots are not always found upon the surface but may lie deep within the 
nacre, to be brought out in the finished button by the processes of shaping and polishing. 
Spotted shells are most common in certain rivers or parts of rivers, particularly where 
the current is sluggish as in partly inclosed sloughs. Some of these discolorations are 
often observed to be associated with a parasitized condition of the mussels, but it is 
not probable that the spots are always due to parasites. The U. S. Bureau of Stand- 
ards, in connection with an investigation of the bleaching of discolored shells, has found 
that the dark-yellow and brown spots are mud fixed by the nitrogenous organic layer 
which binds together the calcium carbonate, and that the pale-yellow color is apparently 
due to an organic coloring matter in the organic layers. That bureau also reports that 
the color of the pink shells is due to an organic coloring which is not confined to the 
organic layer but permeates the whole shell. 




Fig. 6. 



A shell of the pocketbook, Lampsilis ventricosa, which was recovered after 
having been measured and confined in a wire cage in the Mississippi River for 
two years, four and a half months. The line a, an interruption ring, marks the 
size at the time of measuring. The lines b and c evidently correspond to the two 
periods of winter intervening. The inconspicuous sign of a winter interruption 
prccci'.ing the date of measurement does not appear in the drawing. Natural 
size. (After Lefevre and Curtis.) 



134 BULLETIN OF THE BUREAU OF FISHERIES. 

A Striking fonn of shell associated with the presence of parasites is that with abbre- 
viated gaping anterior margins, the edges being much thickened and in appearance 
rolled outward. The explanation appears to be simply that the parasites check the 
peripheral growth of the forward portion of the mantle, or perhaps, as the result of irri- 
tation, keep the mantle more or less retracted in this portion. The shell being controlled 
in growth by that of the mantle, its forward extension is checked, while growth in thick- 
ness continues. Meantime the valves of the shell, growing normally in other directions, 
are gradually and naturally pushed apart as successive layers are added in the posterior 
portions. In consequence, after a time the valves of the shell cease to meet anteriorly 
when the posterior margins are apposed. The result is a shell of normal dimensions 
behind and below but abbreviated in front, where the edges are disproportionately thick 
and gaping. 

A very familiar form of abnormality is shown by the shells in Plate XII. When a 
single shell of this type is first seen one is inclined to suppose that the deformity is the 
result of a mechanical injury; but when shells marked by almost identically the same 
abnormality are repeatedly found in various places and in different kinds of bottom, it 
becomes evident that the explanation of mechanical injury is not applicable. It is prob- 
able that a parasite checked the growth of the mantle at a particular point, so that, while 
growth of shell continued nonnally both before and behind, it was so retarded at that 
point that a pennanently notched outline resulted. The subject of discolored and mal- 
formed shells is not introduced, however, with the object of definitely explaining them, 
but rather with a view to directing attention to the desirability of further investigations of 
the parasites of mussels, as well as of certain features of the environment of mussels, as 
regards their effects upon the form and quality of shells. 



Bull. U. S. B. F., 1919-20. 



Plate XII. 




■«J 













1 llustrating a peculiar abnormality of not infrequent occurrence among fresh-water mussels. 



Bull. U. S. B. F., 1919-20. 



Plate XI 3 



[See text, pape 139. and compare Plate XXI, fig. 1, 
showing marsupium occupying outer gills only. 
Figures after Lefevre and Curtis.] 




-Three-homed warty-back, ObliQuaria refiexa; marsupium 
occupying middle region of outer gills. 




Fig. 4. — Dromedary mussel, 
Dromus drotnas; marsupium 
occupying only lower border 
of outer gills. Anterior end of 
gill not included iu niarsu- 
pium but overhangs it. 



Fig. 5. — Kidney-shell, PtyLhobmtichus pkaseolus; marsupium occupyiug entire lower 
border of outer gills and much folded. 



PART 2. LIFE HISTORY AND PROPAGATION OF FRESH-WATER 

MUSSELS. 

INTRODUCTION. 

The life histories of fresh-water mussels present features in striking contrast to those 
of other familiar mollusks of our seas and rivers. The American oyster, the clam, the 
quahaug, and the sea mussel cast the eggs out to undergo development while floating in 
the water. The pearly mussels of rivers and lakes, on the contrary, deposit their eggs 
in marsupial pouches which are really modified portions of the gills, and there they are 
retained until an advanced stage of development is attained. This particular feature of 
breeding habit is not, however, unique to mussels. There are clams in coastal waters 
that incubate the eggs in the gills, and the common oyster of Europe displays a similar 
habit; but with all these the larvae when released are prepared for independent life. 
Such is not the case with fresh-water mussels. When the larval mussels are discharged 
from the marsupial pouches, the mother has done all that she can for them, but they 
still want the services of a nurse or foster parent, as it were. Lacking the structure and 
appearance of young mussels, they display a peculiar form designated as glochidium, 
and (with few exceptions) they will not continue to live unless they become attached to 
some fish, upon which for a certain time they will remain in a condition of parasitism. 

During the period of parasitic life the glochidium undergoes a change of internal 
reorganization, or metamorphosis, with or without growing in size. After the change 
is complete and a form somewhat similar to the adult is attained, the young mussel 
leaves the fish to enter upon its independent existence. At this time, or soon thereafter, 
some mussels, but not a great number, differ distinctly from the adult form in bearing a 
long, adhesive, and elastic thread, or byssus, by which they attach to plants, rocks, or 
other anchorage. 

The life history, then, comprises the following five stages: (i) The fertilized and 
developing egg retained in the marsupial pouches of the mother mussel; (2) the glochid- 
ium, which, before liberation, is often retained for a considerable further period in the 
gills; (3) the stage of parasitism on fish (or water dogs) ; (4) the juvenile stage, which may 
or may not be marked by the possession of threads for attachment to foreign objects; 
and (5) the mussel stage, with the usual periods of adolescence and maturity. 

Such in brief is the typical story of the life of a pearly mussel. And yet each 
species of mussel, and there are many, has its own characteristic story, which differs in 
more or less important respects from those of other species. One kind of mussel will 
pass through the stage of parasitism only upon a particular species of fish, while another 
kind acquires the aid of certain other fish. The diversity in life histories also manifests 
itself in such details as in the season of spawning, in the part of the gills in which the 
glochidia are carried, in the duration of the incubation period, in the matter of growth 
in size during parasitism, and in many other particulars. There are even some mussels 
which, like exceptions that prove the rule, undergo complete development without being 
parasites upon fish at any stage. It is advisable, therefore, to treat the several stages 

13s 



136 BULLETIN OF THE BUREAU OF FISHERIES. 

of life history at greater length and with such detail as is necessary to establish an 
understanding of the conditions necessary for the successful propagation of the various 
useful rnussels and for the effective conservation of the mussel resources. 

HISTORICAL NOTE. 

It seems appropriate to remark that the considerable fund of knowledge which has 
been gained in very recent years regarding the diversified life histories of fresh-water 
mussels has been gained very largely as a result of scientific studies which have been 
stimulated by the practical need of conserving an economic resource, and which have 
been pursued preliminary to or in connection with the propagation of nmssels as a 
measure of conservation. To put it in another way, the development of 'the fresh- 
water pearl-button industry has furnished an effective stimulus to biological studies 
of high scientific interest and importance, just as the application of science to studies of 
commercial mussels has rendered a distinct economic service. 

As early as 1695 at least, the glochidium (see text fig. 8, p. 143) was observed in the 
gills of European mussels, and was understood to be the larval form of the mussel, although 
it was not then called a glochidium. Of the further stages of life history, science, as well 
as the public, remained in ignorance for a long time. So wide indeed was the gap of knowl- 
edge that it became possible for a scientific writer in 1797 to advance the theory that 
the little mollusks noted in the gill pouches were not young mussels, but were parasites 
of mussels constituting a genus and species of their own, which the investigator designated 
with the Latin name Glochidium parasiticum. This view, known as the Glochidium 
theory, though it never won full acceptance, was strongly supported, and an exhaustive 
inquiry and report upon the subject by a special committee of the Academy of Sciences 
in Paris, completed in 1828, failed to effect its decisive defeat. When, however, in 
1832, Carus was fortunate in observing the passage of the eggs from the ovary of the 
mussel into the gill pouches, the false theory was definitely overthrown. The name 
glochidium, suggested though it was by an erroneous assumption, has persisted ever 
since, being now correctly understood to designate not a distinct animal but a typical 
stage in the development of the mussels. 

It stni remained to determine how and where this peculiar larva became trans- 
formed into the familiar adult mussel, and this important gap was abridged by Leydig, 
in 1866, when the glochidium was discovered in parasitic condition upon the fin of a fish. 

The advance in knowledge of the life history of fresh-water mussels made in the 
ensuing decades was slow and inconspicuous, and textbooks, both American and foreign, 
continued to reproduce accounts based upon the inadequate observations of the life 
histories of European mussels. A period of distinct progress came with the extensive 
and admirable investigations conducted by Lefevre and Curtis (1910, 1910a, and 1912) 
in association with the Bureau of Fisheries during the years 1905 to 191 1. These inves- 
tigations served to reveal not only some of the distinctive features of the breeding 
habits and life histories of the American mussels as contrasted with the European 
species but also the great diversity existing among the many American species, in breed- 
ing season, period of incubation, and form of glochidia. The results of the investiga- 
tions aggregated a mass of original observation on various phases of the propagation 
and life history of fresh-water mussels. Other investigations, notably Ortmann's (191 1, 
1 91 2, etc.), have contributed materially to knowledge of the breeding characters and 



FRESH-WATER MUSSELS. I37 

habits and the development of mussels, while Simpson (1899, 1900, 1914, etc.), Walker 
(1913, 1918, etc.), Ortmann (1911, 1912, 1913, etc.), and others have greatly extended 
our information regarding classification, distribution, and structure. 

With the establishment of the Fisheries Biological Station at Fairport, Iowa, and 
the beginning of its scientific work in 1908, the studies pursued by the scientific staff 
of that station, in connection with the propagation of mussels, made still further advances. 
Chief among the results of the studies conducted at this station may be mentioned the 
discovery that particular species of mussels are restricted in parasitism to one or a few 
species of fish, the rearing of 3'oung mussels in quantity from artificial infections upon 
fish, the demonstration that the glochidia of certain species of mussels may grow mate- 
rially in size during the period of life on the fish (being, therefore, true parasites) , and the 
obser\ration that one noncommercial species of fresh-water mussel normally completes 
its life history without a stage of parasitic life." 

Finally it should be remarked that one of the most difficult of all gaps to bridge 
was the rearing of young mussels after they leave the fish. Strange as it may seem, 
all attempts to keep alive and to rear the young mussels under conditions of control 
failed of result. Lefevre and Curtis (191 2, pp. 182, 183) recorded the rearing from an 
artificial infection of a single young mussel which attained a size of 41 by 30 mm. In 
1914, however, Howard was successful in rearing over 200 Lake Pepin muckets from 
an artificial infection, when the infected fish were retained in a small floating basket in 
the Mississippi River (Howard, 1915). These mussels attained a maximum size of 3.2 
cm. in the first season; and in subsequent years many of them were reared to maturity, 
the glochidia developed from their eggs were infected upon fish, and a second generation 
was reared to an advanced stage. In that year (1914), too, Shira, using watch glasses 
and balanced aquaria, reared a few mussels from an artificial infection to a maximum 
size of 0.44 cm. in 291 days. In the same year, though from an experiment initiated 
by the senior author in the fall of 191 3, young mussels were reared in a pond, from an 
artificial infection of fish liberated in the pond, to a maximum size in the first season of 
3.5 cm. Some of these mussels at the age of 4 years had attained sizes suitable for 
commercial use in the manufacture of buttons. The same species, Lampsilis luteola 
(Lamarck), known as the Lake Pepin mucket, was used in all of these experiments. 
Subsequent experiments on a larger scale conducted both at Fairport and in Lake 
Pepin are mentioned on a later page. 

AGE AT WHICH BREEDING BEGINS. 

The age at which mussels begin to breed varies with tlie species. There is reason 
to believe that the paper-shell, Lampsilis (Proptcra) Icsvissima, breeds in the same sum- 
mer during which it leaves its host or when just i year of age from the egg. Anodonta 
imbccillis and Plagiola donacijormis apparently breed in the second summer. The small- 
est breeding Ouadrula observed was a pig-toe, Quadrula undata, 30 mm. (about 1.2 
inches) in length, and 4 or 5 years of age as evidenced by the interruption rings. The 
smallest washboard, Quadrula heros, observed in breeding condition was 91 mm. (3.58 

o Lefevre and Curtis (191 1 ) had previously observed and reported the fully developed iuveuile mussels in the gills of Strophitus 
edenlidus. Later, Howard (1914) while showing that the glochidia of that species will become parasitic on fish and undergo devel- 
opment under the usual conditions, discovered that another species, Anodonta imbecUlis. normally develops without the aid of 
fish. (See p. 156, below.) 



138 BULLETIN OF THE BUREAU OF FISHERIES. 

inches) in length and of an estimated age of 8 years. Females of the Lake Pepin mucket, 
Lampsilis luteola, reared at the U. S. Fisheries Biological Station, Fairport, Iowa, were 
found with mature glochidia in the third season of growth, a period of slightly more 
than two years after dropping from the fish. Undoubtedly not all species breed at 
such an early age, and it perhaps takes the heavier Ouadrulas 6 or 8 years to reach the 

breeding age. 

OVULATION AND FERTILIZATION. 

With a few exceptions," the sexes are separate in American species of fresh-water 
mussels. The discharge of eggs (ovulation) has been observed in some instances (Latter, 
1891; Ortmann, 1911, p. 298; and Howard, 1914, p. 35). The eggs pass from the 
ovaries by way of the oviduct, through the small genital aperture into the cloaca and 
suprabranchial chambers, and then into the portions of the gills which are to sei-ve as 
brood pouches. The sperm which has been thrown out into the water by one or more 
male mussels, doubtless those in the near vicinity of the female, is taken in by the female 
with the respiratory current, but whether the eggs are fertilized while on the way to 
the brood pouches or after reaching them is unknown, since the process of fertilization 
in nature has never been observed. We have no clue either as to the nature of the 
stimulus which may excite ovulation or as to how it may be timed so as to take place 
when a supply of living sperm is available in the water for the fertilization of the eggs. 
Certain it is that the eggs are usually fertilized, although in the brood pouches of any 
gravid mussel that may be examined there are found a good many eggs that have failed 
to develop, presumably because they have escaped fertilization. 

The discharge of sperm in great quantities may not infrequently be observed when 
male mussels are retained in aquaria. The writers have obser\^ed in a large tank at the 
Fairport station a male mussel discharging sperm. During the process it traveled exten- 
sively over the bottom, leaving in the sand a long winding furrow which was filled with 
a white cloud of sperm. Perhaps the discharge oi sperm and its introduction with the 
respiratory current into the female constitute the exciting cause of ovulation. Exper- 
iments are clearly wanted to determine this question. The arrangement of the eggs 
in the several chambers of the brood pouches varies according to the character of the 
pouch, and will therefore be more conveniently described in the following section. 

BROOD POUCHES OR MARSUPIA. 

The gills of mussels, as of other lamellibranch moUusks, are thin flaps that hang 
like curtains from each side of the body, a pair on each side. As explained in another 
place (p. 175) each gill, thin as it may appear, is really a double structure, or more cor- 
rectly is a sheet folded upon itself just as a map, larger than the page of a book in which 
it is bound, is folded on itself. There is this difference; the map may be unfolded at 
will, but the gill may not, because the two sections are attached together by many par- 
allel partitions which divide the narrow space between the sheets into a lot of long 
slender tubes. It is into these tubes that the eggs are deposited, and when filled with 
eggs or glochidia the several tubes are greatly distended (text fig. 7). The entire gills 
or the parts of the gills bearing the eggs then appear not as thin sheets but as thick 

" The known exceptions are. occasionally, Quadrula rubiginosa and pyramidata, and Lampsilis parva, and. usually, Anodonta 
imbedtlisand henryana (Sterki, 1898), and Sympkyncta compressa and Tiiridts (Ortmann, 1911. p. 308). 



FRESH-WATER MUSSEI^S 



139 



nc. 



pads. In this condition the marsupial pouches might be compared to pods filled with 
closely packed beans, the individual beans representing not single eggs but separate 
masses of eggs. 

When the tubes of a mature female mussel are empty the gills may be as flat as 
those of the males, or they may appear as sacks with thin translucent walls. The lat- 
ter condition generally characterizes the long-term breeders, in which the portions of 
the gill intended to receive the eggs are permanently enlarged. 

The marsupia are conspicuously colored in some species, but in different species 
the coloration is not necessarily attributable to the same cause. In the niggerhead, 
Quadrula ebcnus, the pig-toe, Qiiadrula undata, and other species, the bright-red appear- 
ance of the marsupia is due to the deeply colored eggs showing through the thin walls 
of the marsupia. In the yellow sand-shell, Lampsilis 
anodontoidcs , the pocketbook, Lampsilis vcnincosa, 
and the Lake Pepin muckct, Lampsilis luteola, the 
pigment lying in the outer walls of the ovisacs takes 
the form of dark bands on the lower portion of the 
marsupium, the pigmentation becoming more dense 
and conspicuous when the mussels are gravid. In the 
young Lampsilis cllipsijormis that we have seen the 
pigmentation is more intense and more general, ex- 
tending even to the upper portion of the marsupia, 
but there restricted to the partitions separating the 
ovisacs. The color in the black sand-shell, Lampsilis 
recta, and the Missouri niggerhead, Ohovaria ellipsis, is 
white or cream, in contrast to the yellowish color of 
the remainder of the ovisacs. 

The extent to which the gills are specialized or 
modified to receive and retain the eggs while they are 
developing into the glochidia has been largely utilized 
in the classification of mussels. All of the North 
American species belong to the groups in which the 

brood pouch or marsupium comprises either all four gills or only the outer gills. 
This group, in turn, is divided into the following seven divisions, according to the spe- 
cializations involved (Simpson, 1900, p. 514): 

1. Marsupium occupying all four gills, as in the niggerhead mussel, Quadrula ebenus, and perhaps 
all Quadrulas (PI. XIII, fig. i). 

2. Marsupium occupying the entire outer gills, as in tlie heel-splitter, Symphynota complanala (PI. 
XXI, fig. i). 

3. Marsupium occupying the entire outer gills, but differing from the second in that the egg masses 
lie transversely in the gills, as in the squaw-foot, Sirophitus edentulus. 

4. Marsupium occupying only the posterior end of the outer gills, as in the black sand-shell, Lamp- 
silis recta, etc. (PI. XIII, fig. 2). 

5. Marsupium occupying a specialized portion in the middle region of the outer gills, as in the 
three-homed warty-back, Obliquaria reflexa (PI. XIII, fig. 3). 

6. Marsupium occupying the entire lower border of the outer gills in the form of peculiar folds, as 
in the kidney-shell, Ptychohranchus phaseolus (PI. XIII, fig. 5). 

7. Marsupium occupying the lower border only of the outer gills, Ijut not folded, as in the drome- 
dary mussel. Dromus dramas (PI. XIII, fig. 4). 

Most of the commercial species belong to the first and fotirth types. 
9745°— 21 5 




r.c. 



Fig 



Horizontal section of a water tube of a 

gravid marsupium, showing respiratory canals 
(a. I.), and marsupial space (m. s.), containing 
glochidia. (After I.,efevre and Curtis.) 



140 BULLETIN OF THE BUREAU OF FISHERIES. 

With such species as have all four gills, or the entire outer gills serving as marsupia, 
the sexes are scarcely, if at all, distinguishable from an examination of the shell; but 
when a distinct portion of the outer gill is used as a brood pouch there is usually a pro- 
nounced inflation of the shell over the region of the marsupia, so that the female mussel 
is clearly marked on the exterior. (See also Grier, 1920.) 

It is to be remarked that the eggs packed into the water tubes or marsupial cham- 
bers do not usually remain free of each other, but become either attached together by 
their adhesive membranes or else embedded in a common mucilaginous substance. When 
the eggs or glochidia are removed from the gills they do not separate from one another 
unless fully ripe, but remain in large masses which conform to the shape of the tubes 
from which they have been removed ° (PI. XIV, figs. 8-1 1). It occurs frequently 
when gravid mussels are disturbed that the eggs, in whatever stage of development they 
may be, arc aborted or discharged into the water. This not infrequently happens in 
aquaria, and doubtless may occur in nature. Abortion is presumed to be due to a de- 
ficiency of dissolved oxygen in the water; the mussel, beginning to suffocate, discharges 
the eggs in order to employ its gills more effectively for respiration. 

SEASONS OF DEPOSITION OF EGGS. 

We must distinguish with fresh-water mussels the seasons when eggs are matured, 
passed out of the body, and deposited in the marsupial pouches from the season when 
the developed glochidia are cast out into the water. The tenn "spawning season" 
might be misleading, because it is commonly used to refer to the occasion when the 
glochidia are discharged to the exterior, and this may be weeks, months, or some- 
times nearly a year after the eggs are actually extruded from the reproductive organs 
and the young are launched into existence. In general, the deposition of eggs — the 
actual spawning process, scientifically speaking — occurs with the long-term breeding 
class (see below) in the latter part of the summer or early fall. In the short-term 
breeding class spawning usually takes place in June, July, or August, although in one 
or two species it is known to occur as early as April. One mussel, the washboard, 
deposits eggs only in the late summer and early fall, August to October. 

It is the experience of the Fisheries Biological Station at Fairport that the spawn- 
ing seasons of mussels fluctuate to some degree in different years, no doubt because the 
ripening of mussels is affected by varying conditions of water temperature. There are 
also, of course, some differences of breeding season corresponding to differing climatic 
conditions in more northern or more southern waters. 

SEASONS OF INCUBATION OF EGGS. 

Generally speaking, fresh-water mussels may be divided into two classes with re- 
spect to their breeding seasons — the long-term breeders and the short-term breeders. 

In the case of the long-term breeders the eggs are fertilized during the middle or 
latter part of the summer and, passing into the brood pouches, develop into glochidia, 
which are usually matured by fall or early winter. The glochidia may pass the entire 
winter in the brood pouches, to be expelled during the following spring and early summer. 
As might be expected, there is some overlapping of successive breeding seasons; females 

a Exceptions to this rule are noted by Ortmann (1911. p. 299). In such cases (the genera Anodonta, Anodontoides, Sym- 
phynota, and Alasmidonta) the eggs or glochidia are entirely separate from one another and flow out freely when the ovisac is 
opened. 



FRESH-WATER MUSSELS. 



141 



that have discharged the glochidia quite early in the summer may already have the 
brood pouches filled with eggs for the next season, while other mussels of the same spe- 
cies are still retaining the glochidia developed from eggs of the past year. This fact is 
obviously favorable to the work of artificial propagation, rendering it possible to obtain 
glochidia of certain species of mussels at any time during the year. Thus in Lake Pepin, 
a widened portion of the Mississippi River between Minnesota and Wisconsin, where the 
Lake Pepin muckct or fat mucket is being propagated on a large scale by the Bureau, 
a sufficient number of gravid mussels can be obtained for carrying on the operations from 
the time they are commenced in May until they are tenninated in October or November. 

In the case of the short-term breeders the breeding activities are restricted to a 
season of about five months, from April to August, inclusive. The period of incubation 
for any individual mussel of this class is undoubtedly very much shorter, although tem- 
perature or other conditions may cause the period of incubation to be lengthened or 
shortened. 

In Tables 15 and 16 there are listed the more common species of mussels with indi- 
cation of the months in which females have been found with mature glochidia. The 
lack of a record of gravidity may, of course, be due in some cases not to an actual gap 
in the breeding season but to the want of opportunity for sufficient observation of the 
species during a particular month. (See also Ortmann, 1909; Lefevre and Curtis, 1912; 
and Utterback, 191 6.) 

The commercial and noncommercial species are grouped in different tables, not 
only because the records are more complete for the former but because those who are 
concerned with the conduct or regulation of the mussel fishery will be interested almost 
exclusively in the mussels of direct economic importance. 

Table 15. — The More Important Commercial Mussels, with Indication of Months During 
Which Females Have Been Found with Mature GLOcmDiA. 



Scientific name. 


Common name. 


i 


J3 


i 


a 
< 


i 1 


>> 

3 


< 








>5 


a 
























(?) 

X 

X 
X 


X 
X 
X 
X 






Dromedary mussel 
























yellow sand-shell . . 


X 


X 


X 


X 
X 


X 

X 
X 
X 
X 
X 
X 
X 
X 
X 
X 


X 

X 


X 
X 


X 
X 


X 
X 
X 
X 
X 
X 
























Mucket 




X 
X 


X 
X 
X 
X 


X 
X 
X 
X 
X 
X 


X 
X 
X 
X 
X 
X 
X 
X 


X 
X 
X 
X 
X 

X 
X 


X 
X 
X 
X 
X 
X 
X 


X 
X 
X 
X 


X 

X 
X 

X 




Lampsilis luteola 


Fat mucket . ... 


X 










Pocketbook . 


X 


X 
















X 
X 




X 
X 


X 
X 


X 
X 


X 












Bullhead 






















X 


X 


X 


z 






Rabbit's foot 












X 
X 


X 
X 














X 


X 


X 


X 
X 










Washboard 


X 


X 




X 


X 


X 






X 
X 
X 


X 
X 
X 
X 
X 


X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 


X 
X 
X 
X 
X 
X 
X 
X 


X 






Moakey-face 


























X 












Round Lake 




























X 












Pimple-back 




















do 












X 
X 






















X 
X 

































PiE-toe 










X 
X 


X 


X 




















X 
X 
X 
X 
X 










White heel-splitter 


X 






X 
X 
X 




X 


X 


X 


X 












































X 


X 


















1 











142 



BULLETIN OP THE BtrREAU OF FISHERIES. 



Table i6 —Some Noncommercial Mussels, with Indication of Months During Which Females 

Have Been Found with Glochidia. 



Scientific name. 


Common name. 


i 


i 


i 


a 
< 




i 

l-» 


>. 

3 
1— » 


3 
< 


1 


a 


i 


^ 






















X 








Alasmidonta marginata 

Anodonta cataracta 




X 
X 
X 






X 










X 

X 
X 


X 
X 

X 

X 








X 
X 


X 


X 


X 


X 


X 


(?) 

X 

X 


X 


Anodonta imbcciilis 

Anodonta i mpUcata 










Anodonta suborbiculata 

Arcidcns confragosus 


Paper-shell 

Rock pocketbook 


X 
X 


X 






'.'.'.'. 


X 






X 




X 


X 


Hemilastena ambigua 

Lampsilis alata 

T^ampsilis borealis 




X 


X 


X 


X 


X 


X 


X 


X 
X 
X 
X 


X 
X 


X 








Lampsilis capax 

I,arapsilis cracilis 


Pocketbook 


X 


X 




X 


X 


X 
X 
X 
X 

X 


X 


X 


X 


X 


X 






Lampsilis iris 












X 
X 














Lampsilis Ijcvissima 

Lampsilis lienosa 

Lampsilis parva 




X 




X 


X 


X 


X 


X 
X 




X 








Lampsilis subrostrata 

Lampsilis texasensis 

T flmpsilis veutricosa satura 

Plagiola donaciformis 

Plagiola elcyans 


Deer-toe 










... 


X 
X 
X 

X 


X 


X 
X 


X 










Quadrula cooperiana 

Quadrula granifera 

Strophitus edentulus 


Purple warty-back 

Squaw-foot 




r " 


X 


... 

X 


X 


X 
X 


X 
X 


X 
X 


X 
X 
X 


X 
X 

X 


X 




Symphynota costata 

Symphynota compressa 

Tnmcilla arcseformis 


Fluted shdl 

Sugar-spoon 












.... 


. . * 








Truncilla capsaeformis 

Unio tetralasmus 


Oyster mussel 




::: 






X 


::: 












E 



It will be observed that, generally speaking, the several species of Quadrula and 
Unio, as well as Pleurobema cBsopus (bullhead), TrUogonia iuherculata (buckhom), and 
Obliquaria reflexa (three-horned warty-back) are short-temi breeders, while the species 
of Lampsilis, as well as Obovaria ellipsis (hickory-nut), and Symphynota complanata 
(white heel-splitter), Plagiola securis (butterfly), and others are long-term breeders. 
Most interesting is the case of the washboard, Quadrula heros, which, from its taxonomic 
position, would be expected to have the short summer breeding season, but which at 
least simulates the long-term breeders. The glochidia become mature from early 
autumn to winter, apparently varying with the latitude, but so far as known are not 
held for a long period after maturity. They react like the short-term summer breeders 
when removed from the water in that they quickly abort the contained glochidia. It 
may be either that its relationship has been incorrectly appraised or that it represents a 
transition stage from the short-temi to the long-tenn breeding class. Certainly it is 
the one species of mussel subjected to close study which has never been found to have 
either eggs or glochidia in its gills during the summer months. 

Finally, it may be remarked that the terms "short-term" and "long-term," as 
applied to the breeding season, are perhaps inappropriate and misleading. So far as 
we know, in all species (except the washboard, in one respect) the development of the 
egg into the glochidium follows promptly on ovulation, occupies a period of a very few 
weeks, and occurs during warm weather. The short-term breeders are those which 
throw' out the glochidia at once, while the long-term breeders carry them over until the 
following year. It seems to be a general rule that the short-term breeders pass through 
all phases of reproductive activity on a rising temperature, while the long-term breeders 



FRESH-WATER MUSSElvS. 



143 



begin their breeding activities on falling temperatures of one season, but discharge the 
glochidia on rising temperatures of the folloAving season. 

Several experiments have shown that the glochidia taken from long-term breeders 
in the fall of the year may be successfully infected upon fish and that the young mussels 
will undergo development. It appears, however, that these "green" or newly formed 
glochidia require a longer period of parasitism than those which have been nursed by the 
parent through the winter season (Corwin, 1920). 

The origin and purpose of the retention of glochidia during the winter season re- 
mains a mystery. This may be an instance of nature's remarkable adaptations, per- 
mitting the development of the egg to occur during the warmer months of summer, and 
the glochidia to be discharged for attachment upon fish in the spring when there is a 
general tendency toward an upstream movement of fishes. It is distinctly interesting 
to note that the long-term breeders (mucket, sand-shells, etc.), as a general rule are 
mussels of much more rapid growth than the short-term breeders (mggerhead, pimple- 
back, etc.), although the young of the former are delayed for nearly a year in becoming 
attached to fish and completing their metamorphosis. 

It is important to point out one fact which is clearly established by data in Table 
15, page 141. There is no month of the year in which a considerable number of commer- 
cial mussels are not gravid with glochidia. This fact deserves careful consideration in 
connection with measures of conservation, since it makes impracticable the protection 
of mussels by "closed seasons" of months based upon the times of breeding. 

GLOCHIDIUM. 

The larval mussel or glochidium, when completely developed and ready to emerge 
from the egg membrane and before attaching itself to 
a fish, has apparently an extremely simple organization. 
The soft mass of flesh possesses neither gills nor foot nor 
other developed organ characteristic of the adult mussel, 
but it bears a thin shell composed of two parts which 
are much like the bowls of tiny spoons hinged together 
at the top (text fig. 8). The two parts or valves of the 
shell can be drawn together by a single adductor muscle, 
but, when the muscle is relaxed, they gape widely apart 
as shown in the illustration. There are also on the 
inner surface of each side of the body several pairs of 
"sensory" cells with hairlike projections. It has been 
assumed that the cells were sensory in function, and 
recently L. B. Arey, working at the Fairport station, 
determined after detailed experiments upon several 
species of Lampsilis and Proptera that there is a well- 
developed sense of touch centralized in the hair cells. He regards the tactile response 
as entirely adequate to insure attachment of the glochidium. 

In at least three genera of American mussels (several species of Unio, Anodonta, 
and Quadrula) the glochidium possesses a peculiar larval thread of uncertain signifi- 
cance (text lig. 8). This thread, so generally mentioned in textbooks based upon studies 
of European mussels, is not found on the great majority of American species. We 




Fig. 8. — Glochidium of Quadrula heros with 
gaping valves, seen from a side view. 
The larval thread (/. t.) is seen between 
the valves. Imier and outer sensory hair 
cells (s. h. c.) are visible on each valve. 



144 BULLETIN OF THE BUREAU OF FISHERIES. 

have observed it on glochidia of the following species: The washboard, Quadrula heros, 
the blue-point, Q. plicata, the pig-toe, Q. undata, the bullhead, Pleurobcvia cesopus, the 
spike. Unto gibbosus, the slop-bucket, Anodonta corpulcnta, and the river pearl mussel, 
Margaritana margariiifera. The squaw-foot, Strophitus edentulus, has a modified larval 
thread (Lefevre and Curtis, 1912, p. 173). 

That the structure of the glochidium is less simple than appears to the ordinary 
observer is shown by the fact that, in the fully developed glochidium, close microscopic 
study will reveal the rudiments of foot, mouth, intestine, heart, and other organs which 
will not, however, assume their destined form and functions until after the period of 
parasitism. The shell of the glochidium is firm but somewhat brittle owing to the car- 
bonate of lime of which it is partly composed. If the lime is dissolved out with acid, 
the remaining shell, composed only of cuticle, preserves its general form, although it 
becomes wrinkled and collapsible. 

The number of glochidia borne in the brood pouches of a fully grown female mussel 
according to the counts and computations made by various obser\'ers, varies in the 
different species from about 75,000 to 3,000,000. An example of the paper-shell, Lamp- 
silis gracilis, yielded by computation 2,225,000 glochidia. The mussel was 7.4 cm. 
(about 3 inches) in length. Several examples of the Lake Pepin mucket yielded glo- 
chidia in the following numbers, the length of the mussel being indicated in parentheses: 
(6.1 cm.) 79,000; (7 cm.) 74,000; (7.4 cm.) 125,000; (8.5 cm.) 129,000. 

The glochidia of mussels are very diverse in size and form, although for any given 
species the dimensions and shape of the glochidium have been regarded as fairly con- 
stant (Surber, 1912 and 191 5). Differences in sizes of glochidia within the species are 
noted by Ortmann (1912 and 1919)" and Howard (1914, p. 8). The matter requires 
investigation. As regards their form, glochidia are separable into three well-known types : 
(i) the "hooked" type, {2) the "hookless" or "apron" type, and (3) the "ax-head" type. 

(i) The "hooked" type (PI. XIV, figs, i and 2) possesses a rather long stout hinged 
hook at the ventral- margin of each triangular or shield-shaped valve. These glochidia 
are usually larger than those of the other two types and the shell is considerably heavier. 
The hooks are provided with spines which no doubt assist the glochidium in retaining 
its hold upon the host. As all hooked glochidia generally (though not invariably) attach 
to the exterior and exposed parts of the fish, the fins and scales, the advantage of the 
heavier shell and stout hooks may readily be seen. This type of glochidium is possessed 
by mussels of the genera Anodonta, Strophitus, and Symphynota (floaters, squaw-foot, 
and white heel-splitter, etc.). (See also text figs. 9 and 12.) 

(2) The shells of glochidia of the "hookless" type (PI. XIV, figs. 3, 4, and 5), while 
lighter than those of the hooked type, are nevertheless of sufficient strength to with- 
stand considerable rough handling. So far as we now know, all the glochidia of this 
type are gill parasites with the exception of the washboard, Quadrula hcros, which 
has been successfully carried through the metamorphosis on both gills and fins. The 
hookless glochidia vary rather widely in shape and in size (text figs. 9 to 12); among 
the smallest is that of the spectacle-case, A/argon'towa monodonta (0.05 by 0.052 mm).; 
while one of the largest is that of the purple pimple-back, Quadrula granifera (0.290 by 
0.355 mni.). Placed side by side, about 500 of the smallest or about 80 of the largest 

a Ortmann gives many cases of small discrepancies between his measurements and those of others, based no doubt upon the 
different sources of material. In several cases he has observed differences in sizes of glochidia from different individuals. See 
papers in the Nautilus, Vol. XXVin,i9i4. and Vol. XXJX, 1915. In one instance he reports glochidia of two sizes from one indi- 
vidual C1913, p. 353), Sec also Surber. 1913, p. 4. 



Bui.i,. U. S. B. F., 1919-20. 



PUATE XIV. 














:^:v,:-^::.^ 





V 



iiTJL'i'VI 



11 



[Figures from Lefevre and Curtis, 1912.] 



Figs, i and 2. — Hooked ^lochidiura oi Sym phynota costata. 

Figs. 3, 4, and 5. — Hooklessglochidiumof Lampsilis subro- 
strata. 

Figs. 6 and 7. — Ax-head glochidinm of LampsiUs (Prop- 
tcra) alata. 

Fig. 8. — Conglutinates (masses of glochidia) from the three- 
horned warty-back, Obliquaria rcflexa. 



Fig. 9. — Portion of conglutinate of Obliquaria rcflexa, 
magnified. Glochidia still within egg membranes which 
are closely pressed and adhering together. 

Fig. 10. --Conglutinates (masses of glochidia) from the 
mucket, LaviPsilis lnjamcntina. 

Fig. II. — Portion of conglutinate of Lampsilis Ucameniina 
magnified. Glochidia inclosed in membranes are embedded 
in a mucilaginous matrix. 



Bull. U. S. B. F., 1919-20 




Fig. 3. — Three pill filaments of rock bass, with glochidia of 
mucket. 



T't.i r— Partoffig. I. enlarged. 







Fig. 4. — Stages in formation of cyst surrounding a glochidium of the mucktt. Taken at : -^ inimitt s. ^d minutes, i hour. 

and 3 hours, respectively, after infection. 



'■% 












Fig. 5.— Young muckets, one week after liberation from the fish, showing new 
growth of shell, cilia on foot, and positions assumed in crawling. Enlarged. 

(Figs, i-s after Lefevre and Curtis. 



Fig. 6. — Young Lake Pepin muckets at 
ages of 1. 2, ji. and 4 months, respec- 
tively. Xatural size. 



FRESH-WATER MUSSELS. 



145 















m 



Fig. 9. — Glochidia of commoa fresh-water mussels 

q. Anodo7itoitles fcrussacianus 

subcylin-dr aceus. 
ft, Arcidcns confragosus. 
i, Cyprogenia irrorata. 
j, Dromus dramas. 



n 



(After Surber, 1912 and 1915-) 




a, Alasmidonla calceola. 

b, Alasmidonta marginata. 

c, Anodania corptdenta. 

d, AtwdotUa gratidis. 

e, AnodonlairniieciUis. 



k, Lanipsilis anodontoides. 
I, Lampsilis brei'ictdus brittsi. 
m. Lam P silts fallaciosa. 
n, Lampsilis gracilis. 
0, Lampsilis hifjginsii. 



/ fAnodonta suborbiculata. 



146 



BULLETIN OF THE BUREAU OF FISHERIES. 









m 









u 



Fig. 10. — Glochidia of common fresh-water mussels. (After Surber. 1912 and 1915.) 



a, Lampsilis iris. 

b, Latnpsilis lienosa unicostaia. 

c, L.ainpsilis ligantentina. 
d, Lampsilis luieola. 

e, Lampsilis multiratliaia. 

f, Lampsilis parva. 
p. Lampsilis picta. 



k, Lampsilts recta, 
i, Lampsilis subrostrata. 
j. Lampsilis trabcUis. 
k, Lampsilis venlricosa. 
I, Lampsilis ventr ices a satur a. 
m, Afargaritana monodonta. 
n, Obliquaria reflexa. 



o. Obovaria cir cuius, 
p. Obovaria ellipsis, 
q, Obovaria rctusa. 
r. Plagiola donaciformis. 
s, Plagiola elegans. 
i. Plagiola securis. 
u, Pleurobema cesopus. 



FRESH-WATER MUSSELS. 



147 















m 




n 







Fig. II. — Glochidia of common fresh-water mussels. (After Surber, 1912 and 1915.) 



a and 6, Proptera alaia. 

c, ProPtcra capax. 

d, Proptera laevissima. 
e and/, Proptera purpurata. 

0, Quadrula coccinea, 
h, Quadrula eberms. 



I, Quadrula orani/era. 
j, Quadrula keros. 
k, Quadrula lachrymosa. 
I, Quadrula metanevra. 
m, Quadrula obliqua. 



n, Quadrula plicata. 
o, Quadrula pusiulaia. 
P, Quadrula Pustulosa. 
q, Quadrula solida. 
r, Quadrula undata. 



148 



BULLETIN OP THE BUREAU OF FISHERIES. 









f 

Fig. 12. — Glochidia of common Iresh-water mussels. (After Surber, 1912 and 1915.) 




a, Strophitus edentulus, 

b, Symphynota complanaia. 

c, Symphynota compressa. 



dt Symphynota costata. 

e, Truncilla sulcata. 

/, Tritogonia iuberculata. 



g, Vnio crassidens. 
h, Vnio gibbosus. 



would make a line i inch in length. Hookless glochidia are possessed by practically all 
of the more important commercial mussels; in fact, as far as we know, this type of glo- 
chidium characterizes all the genera and species not mentioned in the paragraphs im- 
mediately preceding and following. 

(3) The "ax-head" type (Pi. XIV, figs. 6 and 7) is considered more closely related to 
the hookless than to the hooked type, although glochidia of this type, except those of 
a single species, Lampsilis (Proptera) Icevissima (Coker and Surber, 191 1), possess four 
hooklike prongs, one at each lower comer of the shell. These pointed projections of 
the shell are not comparable to the pivoted hooks of glochidia of the hooked type. The 
ax-head type of glochidium occurs with the following species: Lampsilis {Proptera) 
(data, Icevissima, purpurata, and capax. (See also text fig. 11, a to f.) 

When the glochidia are fully developed they are ready to break out from the egg 
membrane and to be liberated from the gills of the mussel, although as previously indi- 
cated many species of mussels retain the developed glochidia in their gills for many 
months. A characteristic feature of the mature and healthy glochidium is the active 
snapping together and opening of the shell. This action can be stimulated by adding a 
drop of fish blood or a few grains of salt to the water in which the glochidia are held. 

STAGE OF PARASITISM. 

After the fully matured glochidium has been expelled from the brood pouch of the 
mother, its continued development is dependent upon its coming in contact with the 
gills or fins of a suitable fish host and attaching to them. If it fails to make this attach- 



Bull. U. S. B. F., 1919-20. 



Plate XVI. 




Fig. I.— Filaments of gill of fresh-wau i Inim with heavy natural infection of 
Plagiola donactfonnis. Estimated tuta! number of ylochidia carried by fish 

4,Soo. 





Fig. 2. — Glochidia of washboard mussel, Quadrnla hcros, on 
fin nf fresh-water drum. Cyst very much enlariied. 



Fig. 3.— Section through vacated cysts on gill filaments; 
Qiiadnila ebenus on river herring. 



Bull., U. S. B. F., 1919-20. 



Plate XMI. 






Fig. 2. — A young mussel, Sytnpkynota 
costata, six days after completing the 
stage of parasitism. (Lefevre and 
Curtis.) 



Fig. I. — Glochidium ol Symphynota costata in process of transformation 
during stage of parasitism. (Lefevre and Curtis.) 



r" 





Fig. 3. — A young squaw-foot mussel, Slrop-hitus edenlulm, which had 
completed metamorphosis without parasitism; showing two adduc- 
tor mussels, foot, gills, and rudiments of other organs of adult mussel. 
(Lefevre and Curtis.) 



Fig. 4. — A young mucket, Lampsilis 
ligamentitia. a week after the close of 
the parasitic period. (Lefevre and 
Curtis.) 



FRESH- WATER MUSSEL^. 1 49 

ment it will die within a few days' time. In other words, the glochidium must pass the 
life of a virtual parasite on the fish while undergoing its metamorphosis into the free- 
living juvenile stage. In the light of our present knowledge, this is true of all the fresh- 
water mussels (Unionidae) except the squaw-foot, Strophitus edentulus, and one of the 
small floaters, Anodonta imhecillis. The former species may complete its metamorphosis 
either with or without parasitism (Lefevre and Curtis, 191 1 and 1912, p. 171; and 
Howard, 1914, p- 44), while the latter, as it appears, never endures a condition of para- 
sitism (Howard, 1914, p. 44). 

On coming in contact with the gill filament or fin of the fish the glochidium attaches 
itself by firmly clamping its valves to the tissue of the host. A certain portion of the 
tissue of the fish thus becomes inclosed within the mantle space of the glochidium, and this 
quickly disintegrates and is taken into the cells of the glochidium and consumed as food 
(Lefevre and Curtis, 191 2, p. 169). Within a very short time the tissue of the fish 
commences to grow over the glochidium, presumably in an effort to heal the slight 
wound caused by the "bite" of the glochidium, or perhaps as the result of a positive 
stimulus imparted by the glochidium. L. B. Arey (report in preparation) successfully 

induced encystment by attaching to the 

filaments of excised gills of fish minute .,•■■' j^^ ""-■ 'Vv-x 

metallic clamps the size of glochidia or / ff00^K '■ ' Ft 

smaller. The growth of tissue continues ^" / ' . ; j 'ijiA 

until the larval mussel is completelv -if^^"^ ^ ' ^ 4- -I' .V 

inclosed within a protective covering / ■ i ' ^igs^ i ' I i 

known as the cyst (PI. XVI, fig. 2). \ ° .., ....i^^ 

The several stages of encystment are '■ — xJ./Ai-^^M.., ^ ^^3J '' "" 

clearly represented in the series of fig- „, ,_.j. , ■ , \. , ,•„ , . ■, ,„ ^, ^ 

■^ ^ . Fig. 13. — Glochidium of pmk hecl-splitter. Lampsuts (.PropteraJ 

UreS reproduced from Lefevre and Curtis alala, in condition of parasitism on gill of slicepsliead, showing 

(^1012) (Pi. XV fig. 4) and the process growthofthejuvenilemussdbeyond the bounds of the glochidlal 

maybecompleted within 24 or 36 hours. 

The appearance of a gill bearing a considerable number of glochidia is shown by 
figure I of Plate XV, while figure 2 is an enlarged view of a few of the gill filaments of 
a black bass carrying glochidia of the mucket. 

It is not our purpose to go in detail into the changes which occur in the glochidium 
during the period of its parasitism. They are principally changes of internal structure 
which scarcely affect the external appearance. Nevertheless, at the conclusion of para- 
sitic life the young mussel is a very different sort of an organism from the simply organized 
glochidium which has been described on page 143. Generally it has not increased in size, 
but the single muscle which held the valves of the glochidial shell together has given place 
to two adductor muscles as in the adult; the mouth and the intestine are formed, 
the gills and foot are represented by rudiments which are prepared to function. The 
larval mussel is, in fad , ready to begin its independent life and to take care of itself. All 
of the changes which occur during parasitism require the expenditure of energy and the 
use of body-building material, and as the glochidium enters upon the parasitic life with 
no considerable store of food material, it is reasonable to assume that it derives at least 
a small amount of nutritive material from the fish. Since no growth in size generally 
occurs, the drain upon the fish therefore must be comparatively slight. There are, how- 
ever, a few species (none of the commercial mussels, so far as we know) in which, during 
the period of metamorphosis, the larval mussel grows to a comparatively large size 



I50 



BULLETIN OF THE BUREAU OF FISHERIES. 



(text fig. 13), and, in such cases, the mussel must be generously nourished by the fish. 
(See Coker and Surber, 191 1.) 

The duration of the parasitic period varies greatly with the season of the year during 
which it occurs, and with other conditions which are not fully understood. The results 
of some recent experiments indicate that glochidia of long-term breeders have a rela- 
tively long infection period when they are infected upon fish shortly after maturing 
and a relatively short period when infected after they have remained in the marsupial 
pouches over winter; that is, young glochidia complete metamorphosis in parasitism 

more slowly than old glo- 
chidia. The temperature of 
the water seems to be one 
of the factors governing the 
duration of the parasitic 
period, and doubtless the 
vitality of the host fish is 
another; but there is diver- 
sity even among glochidia of the same species when infected on the same fish. Lefevre 
and Curtis (1912, p. 168), for example, show under such circumstances variations from 
9 to 13 days, and even from 13 to 24 days. The following instances (Table 17) from 
records at the Fairport station are illustrative : 

Tablb 17. — Infections Showinq Duration of Parasitic Period. 




Fig. 14.- 



-A dorsal view of a juvenile pink heel-splitter showing glochidial shell still 
visible. (Xi8). 



Species of mussel. 


Species of fish. 


Date of 
infection. 


Duration 

of 
infection 
in days. 


Average 
water tem- 
perature 
during 
period. 






June 5, 1919 
June 20, 1919 
July 3.1919 
July 9. 1919 
July 23.1919 
June 5. 1919 
June 20. 1910 
July 14, 1919 


13 
12 
II 
13 
12 
13 
13 
10 
II 

13 

10 
12 
12 
12 
11 
10 

IS 

13 
14 
19 

20 

68 
(") 
(«) 
6to S 

9 to II 

11 to 12 
11 

14 to 18 

14 to 2 1 




Do 


do 




Do 


do 




Do 


. . do 




Do 


do 




Lampsilis luteola 






Do 






Do 


do 




Do 


do 




Do 


....do 


.... do 




Do 


do 


July as- 1919 
... do ... 




Do 


... do 




Do 


do 


do 




Do 


.... do 


Aug. 21, 1919 




Do 


do 




Do 


.. do 


Aug. 22, 1919 
June 5. 1919 
June 30,1919 






do 




Do 


do 




Do 


do 




Lampsilis luteola 




July 2; 1914 




Do 






Do 




Aug. 18. 1914 
Sept. 26. 1914 
Sept. 16, 1914 
Aug. 21.1912 
July 7. 1912 
Aug. 4. 191:; 
July 12. 191S 
July 13, 1918 
July 7. 1919 
Oct. 7. 1912 




Do 


do 




Do 










75-1 
78. 1 


Do 




Do 


do 


75-5 
76. 5 
78. 3 






Lampsilis fallaciosa 


do 




....do 




QltnHrnlp Jipfft^ 













n still carrying infection, Apr. 14, 1915. 

In about one week after attachment, as a rule, the wall of the cyst begins to assume a looser texture, 
the intercellular spaces becoming infiltrated with lymph, and from this time on to the end of the parasitic 
period there is little further change in its structure. 

Before liberation of the young mussel, the valves open from time to time and the foot is extended. 
By the movements of the latter the cyst is eventually ruptured, its walls gradually slough away, and the 
mussel thus freed falls to the bottom (Lefevre and Curtis, 1912, p. 171). 



FRESH-WATER MUSSELS. I5I 

Before taking up the history of the mussels in independent juvenile life, we must 
discuss the very significant facts which have been discovered concerning the special 
relation between mussel species and fish species, and refer also to the rare instances known 
of mussels which complete their development without the aid of fish. 

HOSTS OF FRESH-WATER MUSSELS. 

As has previously been indicated in a general way, mussels do not attach to fish 
indiscriminately, but for each species there is a restricted choice of hosts. Some are 
more catholic in their tastes than others, yet for any mussel there is a limited number 
of species of fish upon which it will attach and complete its metamorphosis. The Lake 
Pepin mucket has nine known hosts, while the niggerhead has apparently but one; 
the yellow sand-shell is restricted to gars, and the pimple-back to catfishes. It is, of 
course, employing language in a loose sense to refer to this selection of hosts in terms of 
taste or choice; it is a matter of physiological reaction. When fish and glochidia are 
artificially brought together, glochidia will sometimes attach to the wrong fish, but in 
such cases they soon drop off, or even if partial or complete encystment ensues, the glochi- 
dium does not develop normally and after a time cyst and glochidium are sloughed ofiE 
and lost. It seems evident, then, that successful encystment and development depend 
upon appropriate reactions on the part of both glochidium and fish, and that failure 
ensues upon the lack of a favorable reaction on the part of either parasite or host. The 
reaction may depend in part upon the condition of the individual glochidium or fish, but 
primarily it depends upon the species of mussel and the species of fish. 

It is evident that the artificial propagation of mussels can not be conducted success- 
fully and economically unless we have accurate knowledge of what species of fish serve 
as hosts for the several species of mussels. Such knowledge has been gained by following 
two methods of inquiry, the observational and the experimental. 

By the obser\'ational method, fish taken in the rivers are subjected to careful 
examination for the presence of glochidia on the gills or fins. Preliminary to and 
attendant on such studies, glochidia have been taken from as many species of mussels as 
could be found in gravid condition, these have been studied with the microscope, meas- 
ured, and figured, so that in most cases the species of mussel can be identified in the 
glochidium stage as well as in the adult. (See text figs. 9 to 12.) This method of deter- 
mining the natural hosts is exceedingly laborious. Infection in nature is a matter of 
chance, and only a small proportion of fish bear infections. If it were otherwise, artificial 
propagation might not be necessary. One must, therefore, examine large numbers of fish 
from different localities and at different seasons, and even then the glochidia of some 
species may not be encountered, or they may not be found upon all the hosts to which 
they are adapted. During the calendar year 191 3, for example, 3,671 fish of 46 species 
were examined for natural infections principally during the warmer months from April 
to October. Of these, 324, or 8.9 per cent, were found to be infected with glochidia of 
some species, but only 104 of these, or less than 3 per cent, were infected with glochidia 
of commercial species of mussels'. The fishes infected with commercial mussels belonged 
to 12 species, and the glochidia represented 20 species. The average number of glochidia 
of a given species on infected fish ran from i to 416, with a mean of 125." 

« In August, 1912, 5 examples of the river herring were taken and found to bear glochidia of niggerhead mussels in numbers 
ranging from 1.S95 to 3.740 per fish (Surber, 1913, p. no). Similarly, heavy infections are freciuently found on the fresh-water 
drum, but the glochidia are not usually those of commercial mussels. 



152 



BULLETIN OF THE BUREAU OF FISHERIES. 



The experimental method is simpler in some respects. It consists in submitting 
various species of fish to infection with the glochidia of a given species of mussel and 
observing whether or not the glochidia attach. Since glochidia will sometimes attach to 
fish which are not their natural hosts, it is necessary to hold the fish under observation 
until the mussels have completed the metamorphosis and dropped off. It is, however, 
impracticable to have on hand all the species of fish at the particular time when the 
glochidia of a given species of mussel may be available. Furthermore, the failure of an 
artificial infection to go through successfully on fish held in confinement may be due, 
not to the want of a natural affinity between mussel and fish, but to the fact that the 
fish does not retain its full vitality in close confinement, or to some other defect in the 
experimental conditions. Neither of the two methods for the study of infections may, 
then, be relied upon exclusively for the determination of the natural hosts of fresh- water 
mussels. On the contrary, it has been found necessary to carry on the two lines of study 
hand in hand, according to the plan which was adopted at the beginning of the scientific 
work of the station. In this way, though our knowledge of the hosts of mussels is as 
yet incomplete, there has been obtained a considerable body of information most of 
which is summarized in the following table (i8)," listing 17 species of mussel and 30 
hosts (29 fishes and i amphibian), and indicating those which serve as hosts for each 
species of mussel. 

EXPLANATION OF TABLE i8. 

N. Found on the gills in natural infection. 

Nf. Found on the fins in natural infection. 

n. Record of natural infection but of doubtful significance. 

A. Carried through on gills after artificial infection. 

Af. Carried through on fins after artificial infection. 

a. Results of artificial infection tmsatisfactory or not uniform. 

o. Tested and found unsuitable. 

T. Tested; development occurred; host perhaps suitable, but experiment not carried to conclusion. 

Table 18. — Commercial Mussels and Their Hosts. 



Mussels. 


H 


1 

1 

a 

< 


i 
< 


& 

•s 

< 


.11 
< 


- 

CO ^ 


6 
M 

'0. 

■i 

s 
W 


? 

a. 
si 


a 
1 
s 


1 
1 

§ 

a 


•3 

s 

□ 

g . 

A 

s 




si 
i 

A 

NA 


a 

IS 

li 
80 

j 

A 


3 

3 .a 

If 

no 


J3 


Scientific name. 


Common name. 


e 



c 

g 

1-i 


Lampsilis anodontoidcs . . . 

T.ampsilistallaciosa 

Lampsilis higginsii 

T.ampsilisligamentina 


Yellow sand-shell 








a 



















AN 

























n 


















Mucket 













n 




a 













a 





N 


















n 










a 




Lampsilis ventricosa 







































NA 



AfNf 

















































n 

A 

N 










A 



A 





'n' 


a 

Ni 









Af 


























Quadrula metanevra 




















a 


a 










N 


N 






N 
AN 




A 






Qtiadrula pustulata 


















do 


na 


A 





















































Pig-toe 








































1 








1 




1 









» A great many daU regarding the hosts of noncommercial species ot mussels had been accumulated, but unfortunately most 
of the records applying to such species were destroyed with the burning of the laboratory in December. 1917. 



PRESH-WATER MUSSELS. 
Tabib iS. — Commercial Mussels and Their Hosts — Continued. 



153 



Mussels. 


i 

% 


'5 

3 

1 

►4 


3 
1 

0) 


i| 

.a .3 
1-° 

•§1 
.B 


i. 

.a 

■a g 
■3 XI 


•a 

a 

M 

a 


> 

in 

•g 


1 
s'-g 


a u 

d 

ca 


•3 

s 

3 

1" 


•0 

.9 

u 

P 


•a 

! 

•5s 

is 

.a 3 

to 
.3 

Q. 


•s 
a 

3 

E 


be 

3 

1 
i 


1 

i 
f 


Scientific name. 


Common name. 


Lampsilis anodontoides . 

Lampsilis lallaciosa 

Lampsilis liigginsii 

Lampsilisligamentina.. . 
Lampsilis luteola 


Yellow sand-shell 

Slough sand-shell 


n 









na 










no 

n 


no 


a 




n 


































N 
A 
A 








aN 
NA 
aN 

A 






NA 
on 

a 





N 
A 


AN 
A 






AN 
AN 


NA 
A 

N 

AN 







NA 


A 
A 


AN 
A 








n 




Fat mucket 








NA 


Black sand-shell 










Lampsilis ventricosa .... 
Obovaria ellipsis 


Pocketbook 






A 


A 





AN 






A 











N 















A 




NA 
























Quadrula ebenus 








AN 




n 


TN 
n 





"n" 















Nt 






Quadrula nietanevra 

Quadrula plicata 

Quadrula pustulata 






N 
N 










n 
an 








A 


Nf 

AN 

n 

on 


A 


n 








Warty-back. 




















N 


















n 


























Quadrula undata 


Pig-toe 
















n 


n 









































It will be observed that the number of hosts corresponding to a particular species 
of mussel (as so far determined) varies from one to thirteen. It is of interest to give 
the number of known hosts for each species of fresh-water mussel, as determined both 
by observation of natural infections and by the experimental method, and this is done 
in Table 19. 

Table 19. — Number of Species or Fish Known to Serve as Hosts for Certain Species of 

Mussels. 



Mussels. 



Scientific name. 



Common name. 



Natural 
infection. 



Artificial 
infection. 



Total. 



Lampsilis anodontoides. . 

Lampsilis fallaciosa 

Lampsilis higginsii 

Lampsilisligamentina. ., 

Lampsilis luteola , 

Lampsilis recta , 

Lampsilis ventricosa 

Obovaria ellipsis 

Plagiola securis 

Quadrula ebenus 

Quadrula heros 

Quadrula metanevra. . . . 

Quadrula plicata 

Quadrula pustulata 

Quadrula pustulosa 

Quadrula solida 

Quadrula undata 



Yellow sand-shell . . . 
Slough sand-shell . . . 

Higgin'seye 

Mucket 

Fat mucket 

Black sand-shell 

Pocketbook 

Missouri niggerhead. 

Butterfly 

Niggerhead 

Washboard 

Monkey-face 

Blue-point 

Warty-back 

....do 



Pig-toe. 



(?) 



(?) 



Table 20 lists the com.mon species of fish showing the number of species of mussels 
which each fish has been observed to carry as parasites. The greatest number is six, 
for the bluegill, Lepomis pallidus, the white crappie, Pomoxis annularist and the sauger, 
Stizostedion canadense. 



154 



BUI^LETIN OF THE BUREAU OF FISHERIES. 



Table 20. — Number op Species op Commercial Mussels Known to be Carried as Parasites 

BY Certain Fishes. 



Fishes. 


Natural 
infection. 


Artificial 
infection. 


Common. 




Scientific name. 


Common name. 






Bullhead 


I 

I 
2 
I 

I 
3 

I 
I 

3 

(?) 

5 
I 
I 

(?) 

I 
2 
5 

2 

I 
4 
I 


2 
2 



3 



3 
I 
3 
I 
I 
I 

3 
I 

3 

4 


I 
4 
5 
4 
3 
I 

3 
I 


I 



2 




I 

I 






2 



I 




Anieiiinis iiebulosus . 


do. . . . 






Eel 




Aplodiiiotus grutmiens 


Sheepshead 










Esox lucius 


Pike 










Ictalurus punctatus 


Spotted cat 
















Lepisosteus tristoechus 


Alligator gar 




Lepoinis cyanellus 






l,ci)otnis euryorus 


Sunfish 




Lepoinis hiimilis. 




(') 




Bluegill 
















Micropterus salmoides 










(?) 








































Mad Tom 








6 


pfiyfiiitfdion vitr^urn 


Wall-eye . .. 


I 









f^ An amphibian. 

It is necessary to point to some significant practical conclusions from the data pre- 
sented. Since mussels are "choice" as to their hosts, the chances for the successful 
attachment of glochidia in nature are greatly diminished. The glochidia when dis- 
charged from a parent mussel are lost if no fish are at hand to receive them or if the 
fish that pass are not of one of the very limited number of species which are useful to 
the glochidia of that particular mussel. 

There must necessarily be some definite ecologic relation between the mussel and 
the fish. The bottom that is inhabited by the hickory-nut mussel must be one that is 
frequented by the sand sturgeon during the breeding season of that mussel. Again, if 
one were looking for the river herring, it would be reasonable to expect to find them, 
during June at least, in places where niggerhead beds are known to exist. It is evi- 
dent that no species of mussel could exist unless its host were of such habit as to be at 
the right places at the right times in a sufficient number of cases to perm't first, of the 
infection occurring, and second, of the young dropping where they can survive. 

What the factors are that bring mussels and fish into proper association we can not 
say. In the case of one species of mussel (the pocketbook) at least, it is known that the 
gravid mussel protrudes from its shell a portion of its mantle as a long brightly marked 
flap that waves in the water, assuming the appearance of an insect larva or other at- 
tractive bait (p. 85). Again we have the sheepshead fish (fresh-water drum) which is 
known to feed upon small mollusks, mussels, and the sphaeriids and univalves that live 
on mussel beds, and which thus exposes itself to easy infection ; sheepshead, indeed, are 
almost invariably found to be loaded with glochidia. The behavior of the pocketbook 
is believed to be exceptional, and the sheepshead is one of a very few species of fish 



FRESH- WATER MUSSELS. 1 55 

known to feed directly upon mussels. It is certain, however, that the fresh-water mussel 
beds harbor quantities of other small animal life, such as insect larvae, snails, and 
worms, and are gardens for the food of fishes (p. 119) ; in this, probably, lies the prin- 
cipal clue to the association of fish and mussels. 

Finally, an economic consideration should be emphasized. The conservation of the 
fishes is as important to the preservation of the fresh-water mussel resources and the 
industries dependent upon them as is the propagation and protection of mussels. The 
disappearance, or the radical diminution in number, of certain species of fish would re- 
sult in the complete or virtual disappearance of corresponding species of mussel. On 
the other hand, if the growth of mussels in more or less dense beds produces conditions 
which are favorable to the growth of fish food, and observations do so indicate, then 
the disappearance of the fresh-water mussels would result in the diminution of the 
food supply for fishes, and the conservation of mussels is important for the preserva- 
tion of our resources in fish. 

PARASITISM AND IMMUNITY. 

It is worth while to inquire as to the effect of the glochidia upon fish. Are they 
parasites in the same sense as tapewonns or round wonns ? Do they sap the vitality of 
the fish, and are they accordingly to be regarded as in the nature of a disease? While 
the relation of the glochidium to the fish can not be fully stated in the present stage of 
investigation, it can be said that the principal effect upon the fish, at first, at least, is the 
slight laceration of the gills caused by the attachment of the glochidium. The fish 
quickly heals over this wound to inclose the glochidium and form a small cyst, and 
after that there is in nearly all cases no evidence of further irritation or of material 
detriment to the surrounding tissues, except as the cyst and glochidium are sloughed 
off at the expiration of the proper period. 

The fish feels the attachment of the glochidia; it shows that by the flirting move- 
ments which are made as infection begins, and it is known that excessive infections of 
young fish, at least, may cause the gills to become so lacerated and inflamed as to pro- 
duce the death of the fish (Lefevre and Curtis, 1912, p. 165). The use of small fish is 
avoided in experiments and operations conducted at Fairport, and as care is taken to 
avoid excessive infections it can be said that of thousands of fish artificially infected 
and kept under observation in experimental work at that place there has been no case 
of death or evidently diminished vitality with evidence to implicate the glochidia as 
cause. 

After the microscopic lesion of the gill is healed over, which usually occurs in the 
course of a day, the commercial species of mussels generally make little demand upon 
the fish. No doubt they derive some nourishment from the fish, but this must be very 
slight, since the young mussels, after spending two or three weeks in undergoing meta- 
morphosis, are found to be of the same size as before they attached to the fish." The 
demands upon the energies of the fish caused by the glochidia are probably not greater 
than those arising from a few extra movements. 

It has recently been learned that some fish acquire a certain immunity to glochidia, 
thus being protected against too frequent repetition of infections. Reuling (191 9) has 

a The mussels which grow in size while in parasitism (p. 149) are not commercial species. 

9745°— 21 6 



156 BULLETIN OF THE BUREAU OF FISHERIES. 

found that some of the very large bass, having doubtless experienced some previous 
natural infections, become immune after one heavy artificial infection, while small bass, 
without previous infections presumably, require two or three artificial infections before 
showing immunity. When immunity is acquired, the fish can not be successfully infected 
with glochidia of any species of mussel. The period of duration of immunity is not 
known. 

An earlier significant discovery had been made by C. B. Wilson (1916, p. 341). His 
observations and experiments showed that the fish which are most susceptible to glo- 
chidia are those which are subject to parasite copepods (fish lice) ; that there is a definite 
connection or fellowship of copepods and mussel parasites, so that knowing the species of 
mussel for which a given species of fish serves as host, one may often predict what species 
of copepod fish of that species will carry ; and finally, that the presence of glochidia on an 
individual fish renders that fish practically or completely immune to the attacks of the 
fish lice, and vice versa. These conclusions may be stated in another way: While 
glochidia and copepods have essentially identical taste in fish hosts, the presence of the 
one is antagonistic to the other. 

These observations indicate that artificial infection of fish with glochidia may have a 
positively beneficial effect upon the fish in giving i t protection against a class of parasites 
which are pernicious in effect; for copepods are relatively large parasites which sap the 
vitality of fish and have been known to cause serious mortalities. 

The case of the sheepshead or fresh-water drum, Aplodinotus grunniens, ma}' be sig- 
nificant. Sheepshead are found to be almost invariably loaded with glochidia upon the 
gills, carrying infections which would be regarded as highly excessive if caused artificially 
(PI. XVI, fig. i). They are, no doubt, greatly exposed to infection in consequence of 
the habit of feeding upon molluscs, which they are well fitted to crush with their strong 
grinding teeth. By carrying successfully glochidia, which they secure while devouring 
the parent mussel, they are aiding in the propagation of the mussel which may serve them 
as food. Indeed, the sheepshead unwittingly engages in growing its own food supply. 
Now, of the fish which have been examined in numbers, the sheepshead is the one species 
of fish (besides those of the sucker family, which carry neither glochidia nor copepoda) 
which has never been found to have copepods on the gills. Its immunity from copepods 
is now easily understood, and it may be presumed that this immunity is worth the cost 
of almost continually carrying heavy infections of glochidia. 

METAMORPHOSIS WITHOUT PARASITISM. 

So generally, almost universally indeed, are fresh-water mussels dependent upon fish 
for the completion of their development, that peculiar interest attaches to the two ex- 
ceptions which have so far been encountered. Lefevre and Curtis (191 1) discovered that 
glochidia of one species, the squaw-foot, Strophiius edentulus Rafinesque, may undergo 
metamorphosis into the juvenile stage without the aid of the fish (PI. XVII, fig. 3). In 
this mussel, as in others, the eggs when deposited in the gills are packed in a formless 
mucilaginous matrix, but in the course of the development of the glochidia, the matrix 
becomes changed into the fonn of many cylindrical cords, in each of which a few glo- 
chidia are embedded. There is evidently in this case a special provision for the nour- 
ishment of the embryo from materials supplied by the mother, so that metamorphosis 



FRESH- WATER MUSSELS. 157 

of the glochidium is accomplished at the expense of the parent rather than of a fish. 
Howard (1915) subsequently found that the glochidia of this species could be made to 
attach to fish and would undergo metamorphosis in the usual way on this fish. He also 
discovered that the glochidia of another species, a small floater, Anodonta inibecillis, 
developed into the juvenile mussel within the gills of the parent, and that they would not 
remain attached to fish. 

It is significant that there are just a few species of mussels which diverge in two 
directions from the general rule that fresh-water mussels undergo metamorphosis only in 
parasitism and without evident growth in size during the process. On the one hand, 
we have the cases just cited of change of form accomplished without parasitism, and on 
the other the instances mentioned on page 149 of two or three species in which the larval 
mussel increases many times in growth while still encysted upon the fish. The tendency 
manifested by two species is toward independence of fishes or other hosts, while the 
tendency revealed by a few others is toward a much greater dependence upon fishes. 
The vast majority of species, including all the mussels having shells of commercial value, 
occupy the middle ground of limited dependence upon fish; they must live upon the 
fish, but they require little from them. The hope has been cherished that in time a 
means would be found of supplying artificially to the glochidia of the common species of 
useful mussels the food materials and other conditions necessary for the metamorphosis, 
so that it might become possible to rear mussels without the use of fish. So far, how- 
ever, failure has marked every attempt to accomplish this purpose. 

JUVENILE STAGE. 

At the close of the period of parasite life, the young mussel is no longer a glochidia, 
and while it possesses the rudiments of the principal organs of the adult, it has vet to 
undergo many changes of structure — or better perhaps, a progressive development in 
structure — before it fully assumes the adult form and maimer of life (PI. XV, figs. 5 and 6; 
PI. XVII, fig. 4). To the intermediate stages, or series of stages, between parasitism and 
the development of functional sex organs the term juvenile may properly be applied. 
The siphons or respiratory tubes, the labial palps, outer gills, and sex glands are among 
the conspicuous features of structure acquired during this stage. 

With many and probably most of the common sjiecies of mussels, the early juve- 
nile mussel is no larger than the glochidium — in the case of the L,ake Pepin mucket slightly 
less than one one-hundredth inch in length and slightly more than one one-hundredth inch 
in height. Its thin mussel shell underlies the glochidial shell, and is scarcely visible until 
after several days of growth. The most conspicuous feature of the young mussel at this 
time is the foot, which may be protruded from the shell as a relatively long, slender, and 
active organ of locomotion. The following description applies primarily to the Lake 
Pepin mucket: The foot is somewhat cleft at the apex to give a bilobed appearance and 
it is clothed with ciUa or minute living paddles, which are in rapid motion while the foot 
is extended. The foot has also the power of adhesion to surfaces as smooth as glass; by 
means of it the young mussel can move about rapidly or effect temporary attachments to 
foreign objects. It is not long before the peculiar characters of the juvenile foot are lost, 
for during the first month of independent life this organ becomes changed into the char- 
acteristic form of the foot of the adult mussel. 



158 



BULLETIN OF THE BUREAU OF FISHERIES. 



At a very early stage a special organ of attachment is formed in some species, espe- 
cially among the Lampsilinise (Sterki, 1891, 1891a; Frierson, 1903, 1905; and Lefevre and 
Curtis, 1912). This is the byssus, a sticky hyaline thread produced by a byssus gland 
formed in the middle line of the rear portion of the lower side of the foot. In the wash- 
board, Quadriila heros, a very few days after leaving the fish there is apparent a tough 
mucuslike secretion by means of which the juvenile mussel may anchor itself. The 
byssus may serve to anchor the mussel by attachment to foreign objects, but its func- 
tion needs to be more definitely ascertained. Juvenile nmssels are sometimes captured 
in considerable numbers, owing to the sticky thread becoming attached or entangled on 
the crowfoot hooks or lines or on aquatic vegetation drawn into the boat. While such 
observations suggest the function of keeping the mussel from being carried away by 
the current, nevertheless the organ is well developed in young Lake Pepin muckets 
which are observed to bury themselves deeply in the bottom. The byssus is retained a 
varying length of time in different species and in different individuals of the same species. 
The byssus has been seen in young muckets, Lampsilis ligamentina, late in the second 
year of free life and rarely in adults of Plagiola donacijormis. The species of mussel 
observed with byssus are listed below. 

SPECIES OF MUSSELS THE JUVENILES OF WfflCH ARE KNOWN TO HAVE A BYSSUS. 



I/ampsilis alata, pink heel-splitter. 

L. anodontoides, yellow sand-shell. 

L. capax, pocketbook. 

L. ellipsiformis. 

L. fallaciosa, slough sand-shell. 

L. gracilis, paper-shell. 

L. iris, rainbow-shell. 

I<. laevissima, paper-shell. 

L. ligamentina, mucket. 



L. luteola, Lake Pepin mucket. 
L. recta, black sand-shell. 
L. ventricosa, pocketbook. 
Obovaria ellipsis, hickory-nut. 
Plagiola donaciformis. 
P. elegans, deer-toe. 
Quadrula ebenus, niggerhead. 
Q. plicata, blue-point. 



The shell formed during the first month (more or less) of development possesses 
certain peculiar characteristics — besides having a relatively low lime content and being 
transparent, it bears on its surface certain relatively high ridges, knobs, etc. (PI. XX). 
The cause or the meaning of these nicely formed ridges is unknown, but the pattern of 
sculpture of the early juvenile shell is characteristic for the species. Though all the 
remainder of the shell be perfectly smooth, the "urabonal sculpture," as it is called, can 
be made out in well preserved adult shells of most species, and their markings are given 
significance in the classification of mussels. 

We need not concern ourselves here with the details of development of the internal 
organs, except to say that a considerable elaboration of structure must ensue before 
the mussel is prepared to assume its culminating function — the reproduction of its 
kind. The first act of breeding marks the close of the juvenile period, and this occurs 
in the Lake Pepin mucket two years after the beginning of the juvenile stage, or early 
in the third summer of life counting from the deposition of the egg in the gill of the 
mother. In some species of mussels, those of small adult size, or those possessing very 
thin shells, sexual maturity comes at an earlier age, but in most species of mussels it 
undoubtedly occurs later. (See p. 137.) 

The maximum sizes, at various ages, attained by Lake Pepin muckets under obser- 
vation, are shown in the following table: 



FRESH-WATER MUSSELS. 
Table 21. — Maximum Size ok Young Lake Pepin Muckets at Various Ages. 



159 



Age. 


I^ength. 


Age. 


Length. 




Millimeters. 
0.35 
•5 

4-2 


Inches, 

0. 01 
.02 
■ 17 




Millimeters. 
13.0 
32-3 
58.3 


Inches. 




5 months 






End of second growing season 









This species displays perhaps the most rapid growth of any commercial mussel, 
although it is surpassed in this respect by some of the noncommercial floaters and 
paper-shells. The maximum size attained in the second year by mussels of several 
other species reared at the Fairport station is given in Table 22. 

Table 22. — Size and Age of Mussels Reared at Fairport Station. 



Species. 



Lampsilis ligamentina, mucket 

Lamp-silis anodontoides, yellow sand-shell . , 
Obliquaria refiexa, three-horned warty-back 

Plapiola donaciformis 

Quadrula plicata, blue-point 

Quadrula undata, piy-toe 

Obovaria ellipsis, hickory-nut 



Length. 



MiUimeters. 
20. o 
41.0 
16. o 
20. o 
13- S 
15-8 
II. 4 



Inches. 



Approxi- 
mate age. 



Yean 



Remarks. 



Accidentally reared. 
Intentionally reared. 
Accidentally reared. 

Do. 

Do. 

Do. 

Do. 



Much remains to be learned regarding the habits and habitats of the juvenile mus- 
sels of many species. The study is somewhat difficult, because mussels in the juvenile 
stage are usually hard to find. This is the experience of all collectors, although rich 
finds of larval mussels are occasionally made in particular locations (Howard, 1914, 
pp. 34 and 47). In 1914 Shira collected 1,394 juveniles representing 16 species in Lake 
Pepin, and 92.9 per cent were taken upon sand bottom where there was scattering vege- 
tation. This figure can not, however, be taken as an index of preference for that par- 
ticular sort of habitat, since 86.2 per cent were taken at one station. Isely (191 1 , p. 78) 
made a collection of 32 juveniles comprising 9 species, 6 of which were represented in 
the Lake Pepin collections, but Isely's specimens were all taken in fairly swift water, 
I to 2 feet deep, and from a bottom of coarse gravel. In rearing young mussels, prin- 
cipally Lake Pepin muckets, in ponds at Fairport, the best success has been attained 
on prepared bottom of sand; yet when Howard reared Lake Pepin muckets in a crate 
floating in the river, silt accumulated to a considerable depth, and the juvenile mussels 
were sometimes found deeply submerged in the soft mud ; nevertheless, more than 200 
young mussels survived the season in a very small crate, and excellent growth was made. 

After the byssus is shed the young mussels often bury themselves in the bottom 
more deeply than do adults. They are inclined to travel considerably at this stage, 
but the rate of movement and the distances covered are less than might be thought 
from observation of the conspicuous and apparently fresh tracks behind the young mus- 
sels. It has been found that the tracks will retain the appearance of freshness for sev- 
eral days ; hence the trail which one might at first suppose to have been made in a few 
hours may represent a journey covering a considerable period of time. Clark observed 
a young mussel which made forward movement every 10 seconds, each movement being 



l6o BULLETIN OF THE BUREAU OF FISHERIES. 

followed by a brief rest period. A young hickory-nut mussel was observed to travel 
O.I meter (about 4 inches) in 29 minutes. The rate of travel of sand-shells is much more 
rapid. 

Because of their small size and delicate shell the early juvenile mussels are doubt- 
less the prey of numerous enemies. Turbellarian and chaetopod worms are known to 
devour them. No doubt they are sometimes eaten by fish and aquatic animals, such as 
are accounted enemies of larger mussels, yet there has been found little evidence of 
serious depredations upon young mussels by such animals. Perhaps the most serious 
natural mortality among juvenile mussels occurs from falling upon unfavorable bottoms 
or from the effects of currents, especially in times of flood, which may draw the rela- 
tively helpless mussels into environments in which they have small chance for survival. 
It may be expected, too, that the repeated dragging of crowfoot bars over favorable 
mussel bottoms works damage to juveniles both by injuries directly inflicted and by 
pulling them from the bottom and exposing them to the action of currents from which 
they had previously found protection. 

ARTIFICIAL PROPAGATION. 

PRINCIPLE OF OPERATION. 

As the previous account of the Ufe history of fresh-water mussels has shown, the 
mussel not only deposits great numbers of eggs but nurtures them in brood pouches 
within the protection of her shell. There is not, as in fish, a great wastage of eggs and 
larvae in the very earliest stage of development. There exists, therefore, no necessity 
for artificial aid to effect fertilization ; that is, to bring the male and female reproductive 
elements together. Nature's own provisions have adequately provided for the bringing 
of enormous numbers of each generation of offspring to the glochidium stage. It is 
after this stage is attained that the greatest mortality occurs; the great abundance of 
glochidia produced by • each female is, indeed, evidence that enormous losses are to 
occur subsequently, and observation indicates that the critical stages are, first, when the 
glochidia are liberated from the parent to await a host, and, second, when the juvenile 
mussels are dropped from the fish that serves as host. 

The artificial propagation of mussels as now practiced aims to carry the young 
mussels through the first great crisis. Its object is to insure to a large number of 
glochidia the opportunity to effect attachment to a suitable fish. Under present 
conditions the operations can be conducted extensively and economically only in the 
field. The procedure in brief is to take fish in the immediate vicinity of the places to 
be stocked, infect them with glochidia of the desired species of mussels, and liberate 
them immediatel}'. Artificial propagation, then, as applied to fresh-water mussels, is 
a very different sort of operation from that employed in the propagation of fish, 
although it is no less directly adapted to the conditions and needs of the objects to be 
propagated. 

METHODS. 

In each field the operations are conducted under the immediate direction of a qual- 
ified person who may be either a permanent or temporary employee of the Bureau work- 
ing under the Fairport station. The fishing crew is comprised of three or four local 
fishermen, or laborers, temporarily employed. 



FRESH-WATER MUSSELS. l6l 

The equipment for seining and handling the fish consists of a motor boat, one or 
two flat-bottomed rowboats, seines or other nets, including small dip nets, tanks, 
buckets, etc. The motor boat is used to cover the various fishing grounds as rapidly 
as possible to distribute the infected fishes, and to move the outfit from place to place 
as it becomes advisable or necessary to extend the field of operations. The rowboat is 
employed in the actual work of seining and handling the fish. If the fish are taken in 
very large numbers it is convenient to have one or two tanks, similar to the ordinary 
4-foot galvanized stock tanks and equipped with handles. Under ordinary conditions, 
tubs serve very well, especially if the fish have to be transported by hand for some dis- 
tance, as is the case when the fish are taken in rescue work from land-locked ponds or 
lakes. At times, when the field of operations is at some distance from a place where 
living and sleeping accommodations can be secured, a camping outfit, or a house boat, is 
used for quartering the crew. The head of the party must be provided wdth a dissecting 
microscope, a magnifying hand lens, and simple dissecting instruments. 

Before an infection can be made, it is first necessary to obtain a supply of glochidia 
of the desired species of mussels. In localities where commercial shelling is actively prac- 
ticed this can be done by visiting the shellers' boats and examining the catch for freshly- 
taken gravid mussels. If it is desired to use the glochidia at once, the brood pouches 
are immediately cut from the females and placed in water; btit if it is desired to use them 
over a period of several days, the gravid shells are purchased and the glochidia removed 
as needed. In locations where shells are scarce, or where little or no commercial shelling 
is done, it is sometimes necessary to hire a sheller to procure the mussels. 

The fish are next sought by means of seines or nets, and when secured are sorted and 
transferred to the tanks or tubs; the fish that are not required for purposes of mussel 
propagation are immediately liberated in suitable waters. When the containers are 
comfortably filled with fish, overcrowding being avoided, the brood pouches of one or 
more mussels, as necessary, are cut out and opened with scissors or scalpel and the 
glochidia are teased out in a small pail or other container f ropi which they are poured into 
the tanks with the fish. Figures i to 4, Plate XVIII, show the seining and infection 
operations in the field. 

The experienced operator can usually tell at a glance whether or not the glochidia 
are sufficiently ripe for infection. If they freely separate when removed from the brood 
pouches and placed in a dish of water, it is usually a sign that a sufficient degree of ripe- 
ness has been obtained. If, however, they adhere in a conglutinate mass and can be 
separated only with difficulty, it is certain indication that they are unsuitable for 
infection; examination with a hand lens in such case will show also that the glochidia 
are still inclosed in the egg membrane, thus revealing their immaturity. If the glochidia 
are fully developed, one can readily determine if they are alive and active by dropping 
a few particles of salt or a couple of drops of fish blood into a small dish containing some 
of the glochidia. It is a sign of maturity and vitality if the valves begin to snap together 
as the salt or blood diffuses through the water. 

After being removed from the brood pouches the life of the glochidia is usually 
rather short, but it is possible to keep them alive a day or two if the water in which they 
are retained is changed at frequent intervals and not permitted to become too warm. 

The operator is guided by his experience as to the quantity of glochidia to be placed 
with a given lot of fish and as to the length of the infection period. The water may be 



l62 



BULLETIN OF THE BUREAU OF FISHERIES. 



stirred from time to time in order to keep the glochidia in somewhat even suspension, 
but in most cases the movements of the fish themselves insure a circulation of the water 
and a general distribution of the glochidia. At intervals individual fish are taken by 
hand or small dip net, and the gills examined with a lens; when, in the opinion of the 
operator, a sufficient degree of infection has occurred, the fish are placed at once in 
open waters, or transferred to other containers for conveyance to a place suitable for 
their liberation. The rapidity with which infection takes place depends upon a variety 
of conditions, such as temperatures of water, kind and size of fish, and activity of glochi- 
dia. Ordinarily a period of from 5 to 25 minutes is sufficient to insure an optimum 
infection. The infection time is usually shorter in warm water than in cold. As basis 
for approximate computation of the number of glochidia planted, several average-sized 
specimens of each species of fish infected are killed and the gills removed for subsequent 
counts of the glochidia attached. The counting is done by the foreman with the aid of a 
microscope and usually in the evening after the close of the field operations of the day. 
The number of glochidia per fish of each species having been determined by the count of 
representative examples, and the numbers of fish of the species being known, the entire 
number of glochidia planted on a given lot of fish is easily computed. The data in detail 
are promptly recorded on form cards pro^'ided for the purpose. The count of total 
glochidia planted is of course only approximate, but the method of count and computation 
described is as accurate as the conditions of operation permit, and it is as precise as the 
methods of count generally practiced in fish-cultural operations. In the long run, the 
actual errors on one side and the other must approximately balance. 

That degree of infection which employs the fish to best advantage in mussel propa- 
gation, without doing appreciable injury to the host, is termed the "optimum infection." 
It varies with the species of mussel and with the kind and the size of the fish. Table' 23 
gives illustrative instances. 

Table 23. — Optimum Infection for Certain Species of Mussel on Several Species of Fish. 



Spwcies of mussel. 


Fish host. 


Number of 


Scientific name. 


Common name. 


Species. 


Size in 

inches. 


glodiidia 
on fish. 




I^ake Pepin mucket 


B'ack bass 


8 
8 
8 
5 
<; 
6 

16 
8 

14 




Do 




White bass 




Do 


do 


V, all-eyed pike 

Bhiei:ill 




Do 


do 




Do 


do 


Crappie 




Do 


do 
















Mucket 


Black bass 






Pimple-back 


Channel catfish 


1, 200 









Incidental to the field work in mussel propagation, valuable results are frequently 
gained in the reclamation of fish from the overflowed lands bordering the various rivers. 
All fishes rescued in connection with propagation work, whether suitable or unsuitable 
for infection, are liberated in the open waters, and under such circumstances the value 
of the fish thus saved in large measure recompenses for the cost of the mussel propaga- 
tion work. 

The operations of mussel propagation as just described serve to carry the young 
mussels through the most critical stage of the life history — to give to thousands the 



Bull. U. S. B. F., 1919-20. 



Plate XVIII. 




l-iL.. I.— btiiiiiiji fish from uverfiow water for infection with glochidia of mussels. 




Fig. 2. — Seining fish in Lake Pepin for mussel propagation. 



Fig. 3. — Transferrin!: fish to infectiun tank. Foreman in 
boat is pouring the glochidia from a can into the tank. 




Fig. 4. — Sorting the fish for infection with glochidia. 



Bull. U. S. B. F., 1919-20. 



Plate XIX. 




Fig, 1. — A floating crate containing fuur baskets in which fish infected with glochidia were placed and young mussels reared. 

(Compare PI. V, lig. 3.) 




1*1G. 2. — Lifting one of the Ijuskttb from ihe trale fur exaniiuatiun and cleaning. 



FRESH- WATER MUSSEIvS. 163 

chance of life that would ordinarily fall only to dozens. As previously pointed out 
(p. 151), an extensive series of observations of fish reveals the fact that but few are 
naturally infected with mussels and these usually in slight degree. The chance that a 
large proportion of the glochidia discharged by any mussel will become attached to a 
proper host is slight, and it is only because nature is prodigal in the production of glochi- 
dia that the various species of mussels can maintain their numbers under natural condi- 
tions. With the disturbance of natural conditions by the active pursuit of a commercial 
shell fishery, nature's fair balance is destroyed, and some compensatory artificial aid to 
the propagation of mussels is rendered necessary. 

It is not presumed that all the vicissitudes of mussel life are removed by the bringing 
together of fish and mussel. Nature undoubtedly exacts heavy tolls at other stages. 
Many of the young mussels on being liberated from the fish will fall in unfavorable 
environments and meet an early death, while those that survive the earliest stage of 
independent life may still be subjected to numerous enemies throughout the juvenile 
period at least. Nevertheless, glochidia of certain species can be planted in such large 
numbers and at such slight cost that, after making due allowance for an extraordinary 
subsequent loss, substantial returns can be expected. That such results do obtain is 
indicated both by experiments to be later described (p. 166) and by common experience 

MUSSEL CULTURE. 

The rearing of young mussels in tanks, in ponds, or (if under conditions of control) 
in the river, may properly be termed "mussel culture," as distinguished from "mussel 
propagation," which, as we have seen, consists in bringing about the attachment of 
glochidia to fish and liberating the fish in public waters. For several years experiments 
in mussel culture have been carried on by the Bureau of Fisheries at Fairport and else- 
where, with a view both to securing information regarding the life history of mussels 
and to testing experimentally the possibilities of culture as a public measure of conserva- 
tion or as a field for private enterprise. At first little success attended these efforts. 
It was found that the mussels could readily be carried through the parasitic stage, but 
that soon after leaving the fish hosts they perished. Apparently there was something 
inimical to the young mussels in the artificial conditions of aquaria, tanks, or ponds, 
although these might be supplied with running water derived from the natural habitat 
of mussels. 

The first reported rearing of mussels under control was accomplished with the 
Lake Pepin mucket in a crate floating in the Mississippi River (Howard, 191 5). Ex- 
periments initiated by the senior author in the ponds at Fairport, Iowa, about the same 
time were also successful with the same species. Subsequently broods of the Lake Pepin 
mucket have been reared from year to year by various methods. Less consistent results 
have been obtained with the following river mussels: The pocketbook, LampsUis ventri- 
cosa, the pimple-back, Quadrula piistulosa, and until recently the yellow sand-shell, 
LampsUis anodonioides, and the mucket, LampsUis ligamentina. Apparently the condi- 
tions required for rearing the Lake Pepin mucket are less difficult to meet under control 
than is the case with the other species mentioned. The reason is, doubtless, that Lamp- 
sUis luteola, being a lake-dwelling species as well as an inhabitant of rivers, is adapted to 
more varied conditions. 

The methods employed in rearing mussels may be designated as follows: (i) The 
floating crate with closed bottom (chiefly used in rivers) ; (2) the floating crate with open 



1 64 BULLETIN OF THE BUREAU OF FISHERIES. 

bottom (chiefly used in ponds) ; (3) the bottom crate ; (4) pen with wooden or box bottom ; 
(5) concrete ponds ; (6) earth ponds ; (7) troughs of sheet metal, wood, or concrete tanks, 
and aquaria. 

(i) The floating crate with closed bottom was devised to meet the special conditions 
of a large river where the level is subject to considerable change, where excessive turbidity 
frequently prevails, and where there is a decided current. To prevent the washing away 
of the microscopic mussels, while permitting the passage of water and food through the 
crate, the crates are constructed of fine-meshed (100 mesh to the inch) wire cloth on a 
wooden frame. The form of the crates and the manner of using them may be under- 
stood from the illustrations (PI. XIX, figs, i and 2). They are described in more detail 
in a forthcoming paper by A. D. Howard. A plant of young mussels is obtained by 
placing infected fish in the crate and removing them after they are freed of the mussels. 
The results with the floating crate have been quite satisfactory with the Lake Pepin 
mucket, and a few yellow sand-shells have also been obtained in them. Other river 
mussels have failed to develop beyond early stages. Good results with river mussels 
would be expected, but it is found that even with the crate floating in the river, the 
conditions within it are not those of the natural habitat of the mussel on the clean 
current-swept bottom of the river. No one has yet devised a container to employ under 
such conditions that would fully answer the requirements. 

(2) The floating crate with open bottom has been used in artificial earth ponds. 
The bottom is actually closed to fish, though open to juvenile mussels, since it is made of 
coarse-mesh wire cloth (i K-inch mesh). The infected fish are kept inclosed until freed of 
glochidia, which fall through the wire to the bottom of the pond. To obtain the mussels 
when developed, the water is temporarily drawn from the pond. Good results have 
been obtained with the Lake Pepin mucket only. 

(3) The bottom crate has been used in studies of growth of larger mussels, by 
Lefevre and Curtis (191 2, p. 180), Coker, and others, and in experiments in pearl culture 
by Herrick (Coker, 1913). It has recently been adapted for the purpose of retaining 
infected fish and securing plants of early postparasitic stages of mussels. The crate 
tests on the bottom of the pond. It may have either a solid bottom or one of screen 
wire which, of course, sinks a little way into the mud covering the bottom of the pond. 

(4) The pen of galvanized netting with wooden floor is adapted to quiet water 
without current. The pen, having walls of wire cloth that extend from the bottom to a 
safe distance above the surface of the water, allows the fish to seek their own range of 
depth and permits the mussels that fall from the fish to remain close to the bottom of the 
pond or lake, as is natural for them. The mussels are collected by raising the wooden 
bottom at the end of the growing season. Excellent results have been obtained in Lake 
Pepin with the Lake Pepin mucket. In the most successful experiment more than 
11,000 living young were secured in one crop in a pen 12 feet square. These were 
liberated from 79 fish which had been artificially infected (Corwiu, 1920). 

(5) Concrete ponds having vertical sides have been planted in the usual way and 
the fish removed with a seine after the mussels have been shed. Some 50 examples 
of a river-inhabiting species, the pimple-back, Quadrula pustidosa, were reared to the 
age of 4 years in one experiment, but other trials with this species have failed. The 
usual consistent results have been secured with the Lake Pepin mucket. 

(6) Earth ponds with devices for control of depth and water supply have been 
stocked with mussels by introducing infected fish. As a rule the fish are not removed 



FRESH-WATER MUSSELS. 



165 



until the end of the season when the pond is drawn. The Lake Pepin mucket in con- 
siderable numbers have been reared in earth ponds. A few pocketbook mussels, L. 
veniricosa, were obtained after a recorded plant in a pond of modified type, having earth 
bottom. but wooden sides. Mussels of several other species have been found in ponds 
f 10m accidental plantings. The sporadic occurrences of young mussels in the first ponds 
and in the reservoir constructed at the Biological Station at Fairport, Iowa, are of 
interest as showing how, through parasitism upon fish, many species of mussel will 
quickly invade new waters. It is significant that none of the species which have intro- 
duced themselves abundantly into these ponds are commercially valuable. Apparently 
the commercially useless mussels are more easily and abundantly distributed by natural 
means than the useful ones. A list of the species noted, with additional data, is com- 
prised in the following table (cf . PI. XX) : 

Table 24. — Mussels Recorded from Ponds at the Fairport Station. 



Scientific name. 



Common name. 



Number or frequency. 



Length in 
millimeters. 



Anodonta corpulenta Cooper 

Anodonta suborbiculata Say « 

Anodonta imbecillis Say 

Arcidens conf ragosus Say " 

LampsiJjs ligamentina Lam 

Lampsilis (Proptera) alata Say 

Lampsilis (Proptera) capax Green. . . 
Lampsilis (Proptera) Isevissima Lea. 

Lampsilis subrostrata Say f^ 

Lampsilis gracilis Barnes , 

Lampsilis parva Barnes '^ , 

Obliquaria reflcxa Rafinesque 

Plagiola donaciformis Lea , 

Quadrula plicata Say , 

Quadrula undata Barnes 

Strophitus edentulus Say ^^ , 

Symphynota complanata Barnes 

Obovaria ellipsis Lea , 



Floater 

Paper-shell 

--..do 

Rock pocketbook. 

Mucket 

Pink heel-splitter. , 

Pocketbook 

Paper-shell 



Abundant . 



Abundant. 

7 

7 



Paper-shell. . 



Three-homed warty-back . 

Deer-toe 

Blue-point 

Pig-toe 

Squaw-foot 

White heel-splitter 

Hickory-nut 



Abundant. 

do 

do 

do 



Abundant . 



60-90 
67.4 
a-48 

39-49 
6-20 

69. s 

49- S 

37-90 

8. 43 

9. 1-71 

5-7-27 

16 

2.6-30 

13-5 

IS- 8 

6a- X 

64-91 

11-4 



^ Uncommoa in the river. 

(7) Experiments have also been made with various containers of small dimensions 
which are usually supplied with running water- Such are the glass aquarium and the 
tank or trough which may be made of wood, concrete, or sheet metal- Of these the one 
most used for experimental rearing of mussels at Fairport, Iowa, has been the trough of 
sheet metal painted with asphaltum. A special arrangement for water supply is em- 
ployed. The water is not taken directly from the main reservoir, but is drawn from the 
surface of a pond containing vegetation ; in some cases it is also strained through cloth. 
In this way water is obtained that is very clear and probably free to a large extent from 
such small animals of the bottom as would prey upon the young mussels- The Lake 
Pepui mucket, the river mucket, and the yellow sand-shell have been reared through the 
first year in such troughs- The experiments are of such importance as to merit detailed 
description. The following account is based upon a report of F- H- Reuling, who first 
assisted in the experiments and later was charged with their conduct. (See also Reuhng, 
1919-) 

The experiments were conducted in a series of eight galvanized iron troughs, placed 
at a sufficiently low level to receive a gravity supply of water from pond iD. This pond 
was supplied by gravity from the reservoir which received its supply direct from the 
Mississippi River through the pumping plant- The water in pond iD remained com- 
paratively clear throughout the season, and this was one of the primary considerations 



l66 BULLETIN OF THE BUREAU OF FISHERIES. 

it) locating the troughs. The troughs were 12 feet long, i foot wide, and 8 inches deep, 
painted with asphaltum, and each had its independent inflow from a common screened 
supply pipe in the pond. The bottom of each trough was covered with fine sand to a 
depth of about one-half inch. 

Records were kept of the progress of the larval mussels through the process of devel- 
opment, and when they had reached that stage when they were ready to drop from the 
fish, counts on the fish gave a close approximation of the number dropped in the trough. 

The results of the experiments the first season were quite meager, as only 7 young 
of the Lake Pepin mucket, Lampsilis luieola, varying from 6 mm. to 17.8 mm. in length, 
and 4 of the mucket, L. ligamenHna, with an average length of 2.6 mm., were reared. 
However, in case of the mucket the results were very encouraging, as it marked the first 
instance of juveniles of this species being artificially reared to this size. 

During the season of 191 8 greater results were obtained with the Lake Pepin mucket, 
the young mussels being successfully reared in four troughs. In one trough a count of 
746 was obtained. The experiments with ligameniina yielded negative results, though 
a lack of glochidia for infection greatly handicapped the work with this species. 

The results in 191 9 were still more gratifying. Young Lake Pepin muckets were 
obtained in each of five troughs planted with this species. In one trough 2,008 were 
counted at the end of the season, these little mussels varying in length from 9 mm. to 
17.5 mm., the growth comparing very favorably with that made by the young of this 
species in their natural habitat. In a trough devoted to the river mucket, L. ligamentina, 
a total of 565 were reared. These little mussels varied in length from 5 mm. to 8.5 mm. 
In a trough planted with the yellow sand-shell a count of 2,006 was obtained at the end 
of the season, the young mussels varying in length from 5.5 mm. to 12 mm. The result 
of this experiment is highly interesting, in that it is the first record of the artificial rearing 
of this very valuable species in any quantity. 

The 746 young luteola reared during the summer of 1 91 8 were carried over the winter 
in a shallow crate bottom 5 feet square and 8 inches deep, submerged in one of the earth 
ponds. During the summer of 191 9 an inventory of the crate bottom gave a count of 
238 young mussels, a survival percentage of about 32 per cent. 

The method of artificial rearing of young mussels, as detailed above, denotes a 
distinct departure from the methods previously used and gives the operator complete 
control of conditions throughout. The results of the experiments have been such as to 
justify the employment of the method on a much larger scale in future, and plans are 
under way for materially increasing the facilities and equipment. Certain phases of the 
work need further study and amplification. Additional information on the possible 
enemies of the young mussels in the troughs is needed; a study of their food should be 
made; it should be learned if artificial feeding is practicable; and further experiments 
should be made to determine the most favorable bottom material for the troughs, 
whether fine sand alone, or sand with a slight admixture of silt, etc. The present indi- 
cations are that fine sand is the most desirable bottom material. 

In summary of the topic of the culture of fresh-water mussels, it may be stated that 
the results of many experiments conducted under diverse conditions demonstrate that 
the valuable Lake Pepin mucket can be reared in quantities, under conditions of control. 
Sufficient success has been attained with other species to warrant confidence that, with 
them also, methods of securing constant results will be found. 



Bt'i.i.. U. vS. B. F., 1919-20. 



Plate XX. 







|g^ ^^ 











|fl|b flQ^ VB ^^Ir 

Juveniles of 20 species of mussels found in the artificial ponds at the U. S. Fisheries Biological 
Station within two years from the time of construction of the ponds. Reading from left to right 
these are; 

Top row: Anodonta imbccillis, Anodauta corpulenta, Anodonia suborbiculata, Arcidcns confragosus. 

Second row: Strophitits edentulus .Symphynota complanaia, Lampsilis alata. Lamp si lis laciisiiina. 

Third row: Lampsilis capax, LarnPsilis gracilis, Lampsilis reitlricosa. Lampsihs luteola. 

Fourth row: LamPsilis subroslrata, Lampsilis Parva. Lampsilis ligatnentina. Oborana ellipsis. 

Fifth row: Plagmla donatijormis. Oblig-iiarta reflexa, Quadrnla plicata, Quadrula undata. 

All reproduced natural size excepting the two right-hand figures in top row which are reduced 
one-half. (Photographed by J. B. Southall.) 



Bull. U. S. B. F., 1919-20. 



Plate XXI. 



Ant(iri©r ^ 

retractor mus&fc 
Antarior 
adductor mu&cU' 
Trotractor tnusclii; 
fosition 0/ mouth' - 
(bacK batwaan / 
Labial palp^) / 




OuIerUft labial palp 
Inner U j 1 lals.al palp Visceral mass 



Lunula 

Uefl manlle- 

fg^I*&rior r<;^tracJor mui>c\<i. 
fQ^Xt^riotr adductor muaclc 

^wura-brantniai 
chamber 



RigUt mantle, 
Owtsr ri^nt ^tll 
inn<ir right ^ill 
inner le- j t ^til 



Fig. I. — The structure of a fresh-water mussel. (Based upon drawing of white heel-splitter. Sytnphynota tomManata. 

from Lefevre and Curtis, 1912.) 




Fig. 3.- 



-A tool which, if employed with care, may be used for partially opening livinjj mussels for examination cf conditions 

within the shell. 



PART 3. STRUCTURE OF FRESH-WATER MUSSELS. 
INTRODUCTION. 

A general description of the structure of fresh-water mussels may assist those 
without special knowledge of the anatomy of mussels to follow intelligently the account 
of the natural history, propagation, and development which it has been the primary 
purpose of this report to give. It may also serve as a helpful introduction to persons 
with limited technical knowledge who wish to make original observations or experi- 
ments concerning the habits and growth of mussels. It has been the special purpose 
of the authors to point out the more conspicuous gaps in our knowledge of the behavior 
of mussels and their relations to the environment. Many of these gaps can readily be 
bridged by any who will take the trouble to observe painstakingly and repeatedly the 
conditions under which fresh-water mussels live in the streams, lakes, or ponds in one's 
own neighborhood. The species subjected to observation or experiment should of 
course be definitely known, but identifications of species can always be obtained of 
Government agencies or from independent specialists in the study of mollusks. 

In most localities some species of mussels are easily obtainable and observable 
in nature or in aquaria. In rivers of the Atlantic States, generally, the common mussel 
is the Unio complanatus. The more familiar forms in lakes and alongshore in streams 
of the Mississippi Valley and the Great Lakes drainage are the fat mucket Lampsilis 
luieola, " and some of the floaters of the genus Anodonta. Closely related to the fat 
mucket is the mucket, Lampsilis ligamentina, which is common in the Mississippi and 
its tributaries as well as in many streams discharging into the Great Lakes. As a rep- 
resentative type in the simplicity of its fonn and of the sculpture and markings of its 
shell, the mucket serves as the basis of the following general description, except 
as explicit qualifications are made. With more or less modification, the account may 
be applied to whatever species is most readily available. The functions of the organs 
described will generally be briefly indicated. 

Let it be understood first that a living mussel is commonly partly embedded in the 
bottom, with the forward end directed obliquely downward and the rear end upward. 
The "mouth" as understood by fishermen is in reality the double siphonal opening 
in the hinder part of the mussel; the true mouth, through which food is taken into the 
body, is a very small and scarcely discernible opening in the part of the soft body which 
is farthest away from the exposed end of the mussel. 

The fresh-water mussels difi'er markedly in structure from the oyster or the pearl 
oysters which pertain to a different order of lamellibranchs. They are likewise far 
removed from the sea mussels, which lie in a third order. Their nearer relatives are 
the sea clams and the small Cyrenians of the rivers; the sea clams and the little clams 
(Cyrenians) of the rivers are more closely allied to each other than to fresh-water 
mussels. The pearly fresh-water mussels or Naiades comprise two great families, 

" The best commercial type of the mussels of this species is also known as the "Lake Pepin mucket." 

167 



1 68 BULLETIN OF THE BUREAU OF FISHERIES. 

the Unionidae, with which the present paper is concerned, and the Mutelidae of South 
America and Africa. The Mutelidae differ from the Unionidee in some particulars of 
structure, especially in the form of teeth on the shell and in the form of larva, which 
is a lasidium, instead of a glochidium such as has been described above. 

THE SHELL. 

The shell is composed of two parts very similar in exterior aspect, but generally 
differing from each other in interior form. Each portion is called a valve, and the 
two valves are hinged together. 

EXTERNAL FEATURES. 

In form the shell is roughly elliptical, evenly rounded in front, but more or less 
angular behind. The lower or ventral margin is generally evenly rounded, but may be 
arched inward just behind the middle, especially in shells of females. The dorsal or hinge 
margin is rather straight except for the rounded prominence on each valve just in front 
of the middle of the back; this knob, or arched portion of each valve, is called the 
umbo. Where the umbones of opposite valves approximate each other they are more 
or less elevated above the surrounding shell surface to form the beaks. The beaks in 
many species, though not in the mucket, are beautifully sculptured with coarse or fine 
ridges in the form of single or double loops. With the river mucket, beak sculpture 
is entirely wanting, while it can be seen clearly in Symphynota complanata (PI. XX, 2d 
row, 2d fig). Almost every species, if good specimens are available, show some form of 
beak sculpture;" commonly, however, in older specimens the beaks are so much eroded 
that the ridges are hardly, if at all, apparent. 

In some streams scarcely a single example can be found with the beaks preserved; 
in other waters erosion occurs less commonly and the beak markings can be observed 
even in some of the large examples. 

In some cases the resting periods of winter have left distinct marks by color or 
otherwise on the shell, so that rings or zones corresponding to the growth of each year 
are recognizable. The rings of annual growth are not, however, generally recognizable 
on shells having a dark-colored exterior surface. It is also observed that such rings 
may result from other causes than the interruption of growth by the severity of winter. 
(See p. 132.) 

A conspicuous feature of the shell is the prominent ridge, which extends from the 
beaks backward and downward to the posterior ventral angle of the shell. A somewhat 
similar ridge characterizes almost every species of mussels. 

The exterior color of the shell is a most variable character. Generally speaking, 
the body color is a greenish straw, relieved by narrow green rays, very narrow on the 
beaks and widening out toward the lower margin. These rays are a nearly constant 
character in the mucket, but vary in number, in width, in brightness of color, and in 
being continuous or interrupted. The periostracum, or horny covering, of shells grow- 
ing in clear streams is generally much more brightly rayed than that of those in turbid 

o The beak sculpture of young specimens is a very important diagnostic character or means of distinguishing species which 
may closely resemble each other in general form. Compare the yellow and the slough sand-shells, Lampsilis anodontoides and 
LampsUisfaUaciosa, or the pocketbooks, Lampsilis ventrkosa and Lampsilis (.Proptera) capax, which are occasionally distinguished 
by this feature alone. The beak is, of course, the beginning of the shell— the oldest portion. 



FRESH- WATER MUSSELS. l6g 

ones. Young shells are more brightly rayed than old, the rays generally fading some- 
what or wholly disappearing with age. In different localities, and even in the same 
bed, the colors are various, the shells may be nearly uniformly straw-colored or largely 
green; again, a red or rusty-brown color may predominate. The red color without 
is commonly associated with a pink nacre within. The shell may be smooth and glossy 
or roughened by fine lines; a silky appearance may be caused by innumerable fine 
laminae or folds projecting out from the surface of the periostracum. The silky surface 
is characteristic of some species, as the hickory-nut, Ohovaria ellipsis. 

Looking now at the top or hinge of the shell there is seen just back of the beaks a 
long, narrow, tough, leathery, elastic band, the ligament, an important part of the hinge 
mechanism. Just in front of the beak is a small region between the shell valves, which 
is occupied by a similar horny material. This is called the anterior limule, but in the 
mucket it is scarcely developed, being about one-half inch long and very narrow in a 
specimen of 3 inches total length. A posterior lunule may be found just back of the 
ligament. The compressed form of the shell is noticeable in this view. Roughly speak- 
ing, the thickness of a mucket from side to side is about one-third of the length, while 
the width — or height, more correctly — is about two-thirds of the length. 

INTERNAL FEATURES. 

The interior surface of the shell is smooth, white, and lustrous, and usually somewhat 
iridescent in the extreme posterior portion. In color it is white or pinkish in the mucket, 
but in other species it may be salmon or purple. Often the proper color is obscured by 
yellow, greenish, rusty, or salmon-colored stains, resulting from disease, injury, or in- 
clusion of mud in the nacre. The body of the shell is mainly calcareous, being composed 
chiefly of a compound of calcium of somewhat the same chemical composition as marble 
or limestone, but differing in physical structure from either. An account of the struc- 
ture of shell is given in another place (p. 129). 

The conspicuous features of the interior aspect of the shell are the general con- 
cavity of each valve; the deeper beak cavities; the dorsal margin roughened by ridges 
or protuberances known as the "teeth;" two rounded, impressed, and roughened sur- 
faces, one near each end, the adductor m.uscle cicatrices; and a curved impressed line 
parallel to the margin of the shell, extending between the two scars just mentioned. 
This last is the pallial line and marks the attachment of certain muscles of the mantle. 

The two valves, it is noted, are practically identical except for the teeth, which 
instead of being equal in the two valves, correspond to each other in such a way that 
the teeth of one valve fit into the spaces between the teeth of the opposite valve. The 
two valves are thereby interlocked so that they can not slide over each other. Heavier 
teeth characterize the mussels that are adapted to live in strong currents, while weak 
teeth or the total lack of them mark the species that must live in quiet waters. The 
teeth in each valve are of two forms; at the anterior or front end are the stout, rough, 
and somewhat conical cardinal or pseudocardinal teeth ; while behind these, and more 
or less separated from them, are long, narrow, bladelike ridges, the lateral teeth. On the 
right valve there is one lateral tooth which exactly fits into the deep narrow furrow 
between the two slenderer lateral teeth of the left valve. The two valves are practically 
exact mirror images of each other except for the teeth ; accordingly, in species such as the 



I70 BULLETIN OF THE BUREAU OF FISHERIES. 

Anodontas, which are without teeth, the bilateral S3'mmetry is complete. In some 
marine bivalves the two shells are essentially different, as in the oyster, where one is 
concave while the other is flattened and smaller. 

The ligament is composed of two parts; the dark outer layer is inelastic and con- 
tinuous with the periostracum of the shell; while the inner part, comprising the bulk 
of the ligament, is elastic and bears somewhat inappropriately the name of cartilage. 
The elastic cartilage is confined between the inelastic layer above and the firm hinge of 
the shell below. It is compressed when the shell is closed. The natural or relaxed 
condition of the shell is, therefore, open; that is to say, with the valves separated below 
by about one-half inch. Consequently, the shell is kept closed in life only by an exertion 
on the part of the animal. This is accomplished by means of two stout bands of muscle 
fibers, constituting the anterior and posterior adductor muscles, which extend from one 
valve to the other near each end of the shell. These are firmly attached to the shell 
at each end, the places of attachment being the conspicuous rounded impressions pre- 
viously noticed. 

The hinge mechanism is completed by the lunule previously referred to. This is 
a thin horny covering occupying the space between the valves in front of the beak. 
Unlike the ligament behind, it is stretched when the shell is open. The lunule doubtless 
has no especial significance except to serve as a protective covering and to make a firm 
union of the two valves. 

Besides the two adductor impressions and the pallial line, some smaller muscle im- 
pressions are apparent. Such are those of the muscles which draw back the foot, or 
the anterior and posterior retractor muscles. These are small impressions, two in each 
valve, just above the big adductor impressions and in this mussel {LampsUis ligamentina) 
confluent with the latter. The impression of the protractor, or the muscle which aids 
in protruding the foot, is usually quite distinct and just beneath the anterior adductor 
impression. Deep in the beak cavity and on the under surface of the cardinal teeth, 
or the bridge between cardinal and lateral teeth, are small pits which are the points of 
attachment of numerous small muscles that serve to elevate the foot. These last are 
the dorsal muscle scars referred to in systematic descriptions. (See PI. XXI, fig. i.) 

DIVERSITY IN FORM. 

Many modifications of the above description would have to be made for other species 
of mussels. The shell may be pear-shaped as in the niggerhead (Quadrula chenus), or 
nearly circular as in Quadrula circulus; it may be very much inflated as in LampsUis 
capax or in L. ventricosa (the pocketbook), or exceedingly compressed as in Symphynota 
compressa. In some the shell is not only greatly flattened from side to side but also 
extends upward in wings before and behind the beaks. Such species are given locally 
such descriptive names as pancakes, hatchet-backs (LampsUis alaia), or heel-splitters 
{Symphynota complanaia). Some shells are proportionately very heavy, while others, 
included mostly in the genus Anodonta, the paper-shells or floaters, are so thin as to be 
useless for any present economic purpose. The Anodontas, adapted to live in lakes or 
close alongshore in streams, are further characterized by the entire absence of teeth. 

Variations in thickness or in uniformity of thiclcness are important from the stand- 
point of the button makers, and so also are variations in the surface sculpture. Some 



FRESH- WATER MUSSELS. 171 

forms are covered with protuberances or knobs in regular or irregular pattern, thus ac- 
quiring such common names as warty-backs or pimple-backs; while others have strong 
ridges ruiming obliquely across the shell, as the three-ridge, Quadrula uridulata, the 
blue-point, Q. plicata, and the washboard, Quadrula heros. One species, Unio spinosus, 
of Alabama, bears long sharp spines on the shell. Diversity of interior color has pre- 
viously been alluded to. No satisfactory explanation of the colors of nacre has yet 
been offered. Certain species are almost always white-nacred, as the pimple-back, 
maple-leaf, and niggerhead. Others are white or pink, examples of the two colors 
living side by side. Some species have usually a deep purple or salmon nacre, but 
white-nacred shells of the same species may predominate in particular streams. 

Variations in external color are conspicuous in any collection of shells even from 
the same mussel bed. Along with shells of uniform color, light or dark, we find shells 
of glossy surface and brilliantly rayed; the rays may be continuous or variously inter- 
rupted, sometimes composed of small zigzag markings forming striking and fantastic 
patterns. In short, the differences in form and color of shell are unlimited and could 
not be described, even within the limits of a systematic monograph. 

THE SOFT BODY. 

For observation of the body the mussel may be carefully opened by severing the 
adductor nmscles close to one valve, preferably the left, and gently freeing the soft 
mantle from the shell as the knife blade is passed from one end of the shell to the other. 
Removing or bending back the upper (left) valve, the body of the mussel is seen to be 
almost completely enveloped in a thin mantle corresponding to the interior of the shell 
inform and size (PI. XXI, fig. i). 

FORM AND FUNCTIONS OF THE MANTLE. 

The mantle is composed of right and left sheets entirely free from each other except 
along the back where the two sheets are continuous not only with each other but with 
the body as well. The mantle is, in fact, a double fold from the back of the mussel 
draped over the body and lining the shell. A thin wing or dorsal extension of the man- 
tle covers entirely the surfaces of the cardinal and lateral teeth and underlies the liga- 
ment. 

The mantle is not of uniform character throughout but shows a broad border thicker 
than the central portion and somewhat muscular. This border along its inner line is 
attached to the shell through many fine muscle fibers, the attachment of which forms 
the pallial line on the shell. The border is muscular and, therefore, contractile; the 
lower or right mantle, which has not been separated from the shell, will have its edge 
contracted away somewhat from the margin of the valve; generally there is apparent 
a thin film of horny material which connects the edge of the mantle with the extreme 
edge of the shell. It is not infrequently the case that in separating the surface of the 
mantle from the shell a delicate transparent membrane is distinguishable, some parts 
of which adhere to the mantle and some parts to the shell. Unless, therefore, a rupture 
has occurred, the mantle normally is actually continuous at the margin with the outer 
surface of the shell, and probably organically but delicately connected to the inner surface 
of the shell over its entire surface. 
9745°— 21 7 



172 BULLETIN OF THE BUREAU OF FISHERIES. 

The relations of the mantle as observed will have greater significance from a state- 
ment of its functions. Besides supplementing the gills in respiration and serving along 
its border as a sensory organ, a chief function of the mantle is the formation of shell. 
The extreme edge of the mantle secretes the horny covering of the shell, as also the liga- 
ments and lunule, while the remaining mantle surface secretes the calcareous shell. 
For our purpose, accordingly, the mantle is a most significant organ. Diseases or other 
influences affecting the mantle frequently show effects in the shape, color, or quality 
of the shell, and it is in the mantle, probably, that all free pearls are produced. The 
mantle is not, however, the only portion of the mussel capable of forming shell. The 
two adductor muscles pass entirely through the mantle, having direct attachment to the 
shell. While the shell becomes thicker in other parts by the superposition of layer after 
layer of calcareous material from the surface of the mantle, the thickening of the shell 
against the muscles is in some measure, apparently, a function of the muscles them- 
selves. It is not surprising, therefore, that these muscles also give rise to a large number 
of pearl formations, baroques, and slugs, but not, ordinarily, good pearls. No other 
parts commonly give origin to pearls, although it is reported that pearls have been 
found within the body. Baroque pearls and slugs are frequently found in the tissue 
just beneath the hinge line, but this is actually a part of the mantle. 

The shell substance formed by the muscles is called hypostracum, and is largely 
horny in nature. Since each muscle occupies a nearly constant relative position regard- 
less of the size to which the mussel attains, it is evident that in any adult individual the 
muscle traveled in the course of life history from the back to its latest position; the 
hypostracum, therefore, does not occupy a single spot but is a tapering vein passing 
through the nacre from the beak to the position of the muscle at any given time. Simi- 
larly the hypostracum of the pallial line is the margin of a thin stratum of like sub- 
stance which extends from the beak or beginning of the shell and di^ddes the nacre 
into two portions (p. 130). 

The mantle has other functions of great importance. When the muscles are relaxed 
and the shell is gaping, the opening between the valves of the shell is largely closed by 
the apposed margins of the mantle. Nothing can enter between the valves of the shell 
without affecting the highly sensitive border of the mantle and thus giving warning 
to the animal, which may then contract its muscles and close the shell instantly. The 
nerves of the margin of the mantle are not only sensitive to tactile stimuli, but apparently 
are also connected v.dth organs of something like visual function, so that the animal 
may close or open its shell under the influence of shadows or bright light. 

It is the margins of the mantle that surround and fonn the two siphonal openings 
at the hinder end of the shell, through one of which water and food pass into the shell, 
while through the other water passes out, conveying the waste products. The lower 
of these two openings particularh' is protected by projections of the mantle, in the form 
of papillae or fimbriae, which, being ^'ery sensitive, give warning of any objectionable 
character or content of the water. 

OTHER CONSPICUOUS ORGANS. 

Without disturbing the upper mantle two internal organs are distinctly evident. 
The heart is recognized by its throbbing action. It lies at the back just belo^v the lateral 
teeth of the hinge and in front of the posterior adductor muscle. The rate of beating 



FRESH- WATER MUSSELS. 173 

varies in dififerent species and under different conditions but is generally under 20 pul- 
sations per minute. The heart will continue to beat a long time after the shell has been 
opened. Near the anterior adductor is a greenish mass of tissue, the so-called liver 
or digestive gland, surrounding the white stomach. Through the transparent tissue, 
covering the chamber inclosing the heart, another portion of the alimentary tube is 
generally distinguishable. This is the rectum or hinder portion of the intestine which 
passes directly through the heart to discharge just above the posterior adductor muscle. 
The brownish tissue beneath the heart represents the organ of Bojanus, as it is called, 
with functions corresponding to a kidney. 

To distinguish other organs the mantle must be folded back. The muscular mass of 
plowshare iorm and brownish white in color, constituting the anteroventral border 
of the body, is the foot. Several curtainlike flaps are conspicuous. Toward the forward 
end are two large earlike flaps, the labial palpi or lipfolds. They are easily torn in folding 
the mantle back, but if in good condition, it may seen that each of these palps is contin- 
uous, around the front end of the body, with the palp of the opposite side. Immediately 
in front of the body they are very narrow and lie one above and the other just below an 
exceedingly small opening, the mouth, which can be seen only by very careful exami- 
nation. 

The other two folds are much larger and rounded below. These are the gills, which 
extend from the anterior third of the body to the extreme posterior end. The inner is 
slightly the larger. The outer gill is connected above and on the outside to the mantle. 
Folding this one back, it is seen that it is attached also to the inner gill above. The inner 
gill on the inner side is attached to the body and, behind the body, to the inner gill 
of the opposite side. In many species the inner gill is partially free from the body. 
These gills, though thin, are really basketlike structures, containing chambers within, 
as will be described below. 

INTERNAL STRUCTURE. 

It is not the province of this paper to enter minutely into the internal anatomy. 
But the following epitomized statement of the structure of the animal is given to serve 
as a key to the understanding of the functions of the organism as a whole. 

The digestive system comprises the mouth, with a short tube or gullet, leading 
from the mouth to the stomach; the dark brown digestive gland, or so-called liver, 
which surrounds the stomach; and the intestine, which is a long tube that leads down- 
ward from the stomach and coils upon itself behind the foot in a complex way, before 
bending upward to approach the back and extend posteriorly straight through the heart 
as the rectum, which opens just above the posterior adductor muscle. A long, slender 
flexible gelatinous rod, the crystalline style, is frequently found in the intestine; it 
serves a function in separating food from foreign particles and comprises a store of 
enzymes or ferments for use in the processes of digestion (Nelson, 1918). 

The excretory system comprises a functional kidney with a bladder which discharges 
into the cavity surrounding the heart. 

The circulatory system includes, as in higher animals, heart, blood, arteries, and 
veins. The blood of a mussel is colorless but maintains a regular circulation from the 
heart through certain arteries to many smaller vessels ramifying all through the body, 
returning by a main vein to the kidneys, thence to the gills and back through other 
veins to the heart to begin its course anew. The blood, however, which passes from 



174 BULLETIN OF THE BUREAU OF FISHERIES. 

the arteries to the mantle, returns, not through the kidneys or the gills, but directly 
to the heart. 

The mantle and the gills constitute the chief respiratory organs, where the blood 
is aerated. The significance of the mode of circulation is evident. The venous blood 
returning from the body laden with waste products passes first to the kidney, thence 
to the gills to be cleared of impurities and freshened with oxygen, after which it returns 
to the heart in purified condition. The blood returning from the mantle requires no 
further purification or oxygenation before entering the heart. 

Without a distinct brain, the body of the mussel is coordinated through a ner\'ous 
system, consisting of three pairs of nerve centers, which are connected together by 
nerve cords. Two of these centers or ganglia lie one on each side of the gullet near the 
mouth, a second pair is in the foot, while the third lies just beneath the posterior adductor 
muscle. From these ganglia fine nerves are sent off to supply the various tissues 
and organs. 

Though eyes and ears are not present, sensory organs are not entirely wanting. 
A small organ near the ganglia beneath the posterior adductor is supposed to serve to 
test the purity of the water. Another, the otocyst, is sometimes found near the ganglia 
in the foot and possibly serves as a balancing organ, by means of which the mussel 
may feel whether it is in horizontal or vertical position. Sensory cells are found along 
the border of the mantle, especially near the posterior openings for the passage of water. 
(Seep. 87.) 

The organs of reproduction comprise a large part of the body mass above the foot. 
The ova or semen are discharged through small openings on each side of the body into 
the chamber above the gills. In the case of the male the sperms are thence passed 
out with the respiratory (exhalent) current and set free in the water. They may be 
drawn into the female with the water of the inhalent current, to fertilize the ova perhaps 
as they are passed down from the suprabranchial chamber into the tubes in the gills 
where incubation takes place. In some species the reproductive tissue is brightly 
colored — orange, pink, or red. 

STRUCTURE AND FUNCTIONS OF THE GILLS. 

The gills, as the name would suggest, are primarily breathing organs. Nevertheless, 
they have an equal if not a greater function in food gathering, and, furthermore, in 
fresh-water mussels and in some other lamellibranchs, the gills have acquired a third 
office which is of coordinate importance with the other two. We have seen that the 
incubation of the egg takes place in the water tubes of the gills, a part or all of which may 
be filled with embryo mussels. The respiratory function of the gills of the female mussel 
must be greatly reduced during the period of incubation, and this condition is made 
possible by the fact that the mantle of the mussel plays an equal role with the gills 
in respiration. In becoming adapted to this function of protection and perhaps nour- 
ishment of the eggs and young, the gills of the female have undergone varied modifica- 
tions in different species. In consequence, when gravid females can be examined, the 
gills of different mussels are often found to be more strikingly distinct than is the external 
form or any other obvious character. This is especially true when microscopic study 
of the structure of the gills can be made. 



FRESH- WATER MUSSELS. 175 

Whether or not, therefore, these differences are a true guide to relationships, the 
gills become one of the most convenient organs for distinguishing genera or species and 
serve as the most important basis of modern classification. 

Some knowledge of the anatomy of the gills is necessary for proper comprehension 
of the life process of mussels in breathing, feeding, and reproduction. 

The gills consist, as we have seen, of two platelike bodies on each side between the visceral mass 
and the mantle. We have thus a right and a left inner gill and a right and a left outer gill. Seen from 
the surface, each gill presents a delicate double striation, being marked by faint lines running parallel 
with the long axis and by more pronotmced lines running at right angles to the long axis of the organ. 
Moreover, each gill is double, being formed of two similar plates, the inner and outer lamellse united 
with one another below as well as before and behind but free at the top or dorsally. The gill has thus 
the form of a long and extremely narrow bag open above. Its cavity is subdivided by vertical bars of 
tissue, the interlamellar junctions, which extend between the two lamellse and divide the intervening 
space into distinct compartments or water tubes, closed below but freely open along the dorsal edge of the 
gill. The vertical striation of the gill is due to the fact that each lamella is made up of a nimiber of 
close-set gill filaments; the longitudinal striation, to the circumstance that these filaments are con- 
nected by horizontal bars, the interfilamentar junctions. At the thin free, or ventral, edge of the 
gill the filaments of tlie two lamellte are continuous with one another, so that each gill has actually a 
single set of V-shaped filaments, the outer limbs of which go to form the outer lamella, their inner limbs 
the inner lamella. Between the filaments, and boimded above and below by the interfilamentar 
jimctions, are minute apertiu-es or ostia, which lead from the mantle cavity through a more or less 
irregular series of cavities into the interior of the water tubes. (After Parker and Haswell.) 

The gills, then, which appear as thin plates, are really comparable to long baskets 
greatly flattened from side to side, the interior of the basket being subdivided into a 
series of deep tubes, all in one row. The surface of the basket, which is perforated by 
many pores visible only with a microscope, is covered with very minute paddles like 
fine flat hairs. The concerted action of these little paddles, called cilia, keeps driving 
the water from without the gill through the minute pores into the water tubes. Through 
these tubes the water passes upward into a chamber above the water tubes, called the 
suprabranchial chamber, and thence backward and finally out of the shell. 

Since the cilia are habitually driving the water through the surface of the gills 
into the water tubes, it follows that there must be a regular stream of water entering 
the mantle chamber from without through the open valves, as well as an outgoing 
stream passing out from the chamber above the gills. These two streams are known 
as the inhalent current and the exhalent current, respectively. If a mussel is observed 
in undisturbed condition on the bottom of an aquarium (PI. V, figs, i and 2), the two 
openings between the edges of the mantle are readily seen and the currents may easily 
be observed by introducing with a pipette into the water near each opening a little 
colored water. The coloring matter placed near the lower inhalent current is drawn 
into the shell, but that placed near the upper opening is driven forcibly away. The 
two pronounced currents, or rather two aspects of the same current, are, it may be 
repeated, formed entirely by the minute paddles surrounding the innumerable pores 
of the gill surfaces. 

The gills themselves are living strainers in the course of this current, and as the 
water passes through them the material which serves as food is filtered out to be passed 
on to the mouth; at the same time, the blood in the minute vessels and spaces within 
the gill filaments and partitions is being purified and recharged with oxygen. The 
matter strained from the water becomes clotted with mucus and is driven along by the 
cilia over the surface of the gills to the labial palpi, where it is taken up and if suitable 
for food is passed on to the mouth, for the surfaces of the palpi as well as of the gills 



176 BULLETIN OF THE BUREAU OF FISHERIES. 

are covered by cilia or minute paddles, the combined action of which forms a wonderful 
mechanism for conveying the food from any point of the gill surface into the funnel- 
shaped mouth. The detailed working of this mechanism and the places and means of 
"switching off" undesirable matter form too complex a subject to be treated in this 
paper. (See Allen, 1914, and Kellogg, 1915.) 

The course of the water is better understood after observing the mode of attachment 
of the gills. The outer lamella of the outer gill is attached to the mantle throughout 
its entire length, while its inner lamella and the outer lamella of the inner gill are attached 
together to the body. There is thus above each gill a small suprabranchial chamber 
just above the water tubes. Behind the body or visceral mass, however, the inner 
lamella of the right and left inner gills are attached together, and there is, therefore, 
a single large chamber above the four gills — the cloaca or exhalent chamber. The 
water, after passing through the pores of the gill surface, makes its course up the water 
tubes and backward by the suprabranchial chamber into the cloaca, to be passed thence 
out of the shell." 

It will be understood that the eggs and young borne in the water tubes of the gills, 
which become marsupial pockets, are most favorably located for respiration, being 
situated, as it were, in the respiratory current of the mother. There is, among the 
various species of the Unionidae, great variation in the extent to which the gills are 
employed as marsupia (p. 139). In certain species the water tubes of all four gills are 
filled with eggs, in others only those of the outer gills receive the eggs, while in still 
others a portion of each outer gill is set apart as a marsupium. This may be the posterior 
half, the posterior third, or a few water tubes in the middle. 

It is largely because of the great significance of the gills with their remarkably 
diverse functions of food collection, respiration, and gestation that the modifications 
both in the external form and in the histologic structure of the gills are important and 
serv'e so well as a basis of classification. Generally speaking, species in which all four 
gills serve as marsupia are considered lower or more primitive forms. Those in which 
the marsupia are most highly specialized are regarded as most highly developed. 

" The effect of the gills in filtering the water is made clear when one fills two iars with turbid river water after placing in each 
sufficient sand for a mussel to become embedded. If one or two mussels are placed in one of these jars, the water will become 
clear in a comparatively short time. 



BIBLIOGRAPHY.'^ 

Adeney, W. E. 

1912. See New York (City). Report Metropolitan Sewerage Commission of New York. New 
York. 
Ai,i,EN, W. R. 

1914. The food and feeding habits of fresh-water mussels. Biological Bulletin, Marine Biological 

Laborator>', Woods Hole, Mass., Vol. XXVII, No. 3, pp. 127-146, 3 pis. Lancaster. 
Baker, Frank Collins. 

1898. The Mollusca of the Chicago area. The Pelecypoda. The Chicago Academy of Sciences, 
Bulletin III, Part I of the Natural History Survey, pp. 9-130, pis. 1-27. Chicago. 

1910. The ecology of the Skokie Marsh Area, with special reference to the Mollusca. Bulletin, 

Illinois State Laboratory of Natural History, Vol. VIII, Art. IV, pp. 441-499. P's. VI-XXV. 
Urbana. 
1916. The relation of moUusks to fish in Oneida Lake. Technical Publication No. 4, New York 
State College of Forestry, Syracuse University, Vol. XVI, No. 21, pp. 1-366, figs. 1-50. 
Syracuse. 

1918. The productivity of invertebrate fish food on the liottom of Oneida Lake with special ref- 

erence to mollusks. Technical Publication No. 9, New York State College of Forestry, 
Syracuse University', Vol. XVIII, No. 2, pp. 1-253, figs. 1-44, 2 pis. Syracuse. 
1920. Animal life and sewage in the Genesee River, New York. The American Naturalist, Vol. 
LIV, pp. 152-161. New York. 
BiRGE, E. A., and Joday, C. 

1911. The inland lakes of Wisconsin. The dissolved gases of the water and their biological 

significance. Wisconsin Geological and Natural History Survey, Bulletin No. XXII, 
Scientific Series No. 7, 259 pages. Madison. 
BoEPPLE, J. P., and Coker, R. E. 

1912. Mussel resources of the Holston and Clinch Rivers of eastern Tennessee. U. S. Bureau of 

Fisheries Document 765, 13 pages. Washington. 
Call, Richard E. 

1900. A descriptive illustrated catalogue of the Mollusca of Indiana. Twenty-fourth Annual 

Report, Department of Geology, State of Indiana, pp. 335-5^5, pis. 1-78. Indianapolis. 
Churchill, E. P., Jr. 

1915. The absorption of fat by fresh-water mussels. Biological Bulletin, Marine Biological Lab- 

oratory', Woods Hole, Mass., Vol. XXIX, No. i, pp. 68-86, 3 pis. Lancaster. 

1916. The absorption of nutriment from solution by fresh-water mussels. Journal of Experimental 

Zoology, Vol. XXI, No. 3, pp. 403-429. Baltimore. 
Clark, H. Walton, and Gillette, George H. 

1911. Some observations made on Little River, near Wichita, Kans., with reference to the Llnion- 

idae. Proceedings, Biological Society of Washington, Vol. XXIV, pp. 63 68. Wash- 
ington. 
Clark, H. Walton, and Wilson, Charles B. 

1912. The mussel fauna of the Maumce River. U. ,S. Bureau of Fisheries Document 757, 72 pages, 

2 pis. Washington. 
Coker, Robert E. 

1913. Demonstration of Dr. Herrick's free pearls of forced production. Transactions, American 

Fisheries Societ)', 1912, pp. 71-74. Washington. 

1919. Fresh-water mussels and mussel industries of the llnited States. Bulletin, U. S. Bureau of 

Fisheries, Vol. XXXVI, 1917-18, pp. 13-89. pis. I-XLVI. Washington. 

a Including only publications cited. 

177 



178 BULLETIN OF THE BUREAU OF FISHERIES. 

CoKER, Robert E., and Southall, John B. 

1915. Mussel resources in tributaries of upper Missouri River. [With description of shell found in 
the James River, Huron, S. Dak., July 27, 1913.] Appendix IV, Report, U. S. Com- 
missioner of Fisheries, 1914, 17 pages, i pi., i fig. Washington. 
CoKER, Robert E., and Surber, Th.'^ddei's. 

1911. A note on the metamorphosis of the mussel Lampsitis l<rvissimus. Biological Bulletin, 
Marine Biological Laboratory-, Woods Hole, Mass., Vol. XX, No. 3, pp. 179-182, i pi. 
Lancaster. 
CORWIN, R. S. 

1920. Raising fresh-water mussels in enclosures. Transactions, American Fisheries Society, 
Vol. XLIX, No. 2, pp. 81-84. Columbus. 
Dangi.ade, Ernest. 

1914. The mussel resources of the Illinois River. Appendix VI, Report, U. S. Commissioner of 
Fisheries, 1913, 48 pages, 5 pis., 2 figs., i map. Washington. 
Dole, R. B. 

1909. The quality of surface waters in the United States. Parti. Analyses of waters east of the 
one hundredth meridian. U. S. Geological Sur\'ey Water-Supply Paper 236, 142 pages, 
I map. Washington. 
EvERMANN, Barton W.\rren, and Clark, Howard Walton. 

1918. The Unionidse of Lake Maxinktickee. Proceedings, Indiana Academy of Science, 1917, 

pp. 251-285. Indianapolis. 
Farrer, W. J. 

1892. Mortality- in mussels at Orange, Va. The Nautilus, Vol. V, p. 141. Philadelphia. 
Forbes, Stephen A. 

1913. Biological and chemical conditions on the upper Illinois River. Proceedings, Fifth Meeting, 

Illinois Water Supply Association, Urbana, HI., pp. 161-170. 
Forbes, Stephen A., and Richardson, Robert Earle. 

1919. Some recent changes in Illinois River biolog)-. Bulletin, Department of Registration and 

Education, Division of the Natural History- Survey of Illinois, Vol. XIII, Art. VI, pp. 
139-156. Urbana. 
Frierson, L. S. 

1903. Observations on the byssus of I^nionid^e. The Nautilus, Vol. XVII, pp. 76-77. Boston. 

1905. Notes on young Unionida;. The Nautilus, Vol. XIX, pp. 49-50. Boston. 
Grier, N. M. 

1920. Sexual dimorphism and some of its correlations in the shells of certain species of Najades. 

The American Midland Naturalist, Vol. VI, No. 8, pp. 165-172. Notre Dame. 
1920a. Variation in the nacreous color of certain species of Naiades inhabiting the upper Ohio, 
drainage and their corresponding ones in Lake Erie. American Midland Naturalist, 
Vol. 6, pp. 211-243. Notre Dame. 
Headlee, Thomas J. 

1906. Ecological notes on the mussels of Winona, Pike, and Center Lakes of Kosciusko County, 

Indiana. Biological Bulletin, Marine Biological Laboratory, Woods Hole, Mass., Vol. 
XI, No. 6, pp. 305-318, I pi. Lancaster. 
Headlee, T. J., and Simonton, James. 

1904. Ecological notes on tlie mussels of Winona Lake. Proceedings, Indiana Academy of Science, 

1903, pp. 173-179. Indianapolis. 
Howard, A. D. 

1914. Experiments in propagation of fresh-water m.ussels of the Quadrula group. Appendix IV, 

Report, U. S. Commissioner of Fisheries, 1913, 52 pages, 6 pis. Washington. 
1914a. Some cases of narrowly restricted parasitism among conmnercial species of fresh-water 
mussels. Transactions, American Fisheries Society, Vol. XLIV, No. i, pp. 41-44. 
New York. 

1915. Some exceptional cases of breeding among the LTnionidas. The Nautilus, Vol. XXIX, pp. 

4-1 1. Boston. 



FRESH- WATER MUSSELS. 179 

IsELv, Frederick B. 

igii. Preliminary note on the ecology of the early juvenile life of the Unionidae. Biological 
Bulletin, Marine Biological Laboratory, Woods Hole, Mass., Vol. XX, No. 2, pp. 77-80. 
Lancaster. 

1914. Experimental study of the growth and migration of fresh-water mussels. Appendix III, 

Report, U. S. Commissioner of Fisheries, 1913, 24 pages, 3 pis. Wasiiington. 
Kellogg, J. L. 

1915. Ciliary mechanisms of Lamellibranchs with descriptions of anatomy. Journal of Morphology, 

Vol. XXVI, No. 4, pp. 625-701, 72 figs. Philadelphia. 
Kelly, H. M. 

iSpQ. A statistical study of the parasites of the Unionidee. Bulletin, Illinois State Laboratory- of 
Natural History, Vol. V, Art. VIII. Urbana. 
Latter, O. H. 

1891. Notes on Anodon and Unio. Proceedings, Zoological Society of London, pp. 52-59. Ixindon. 
LEFEVRn, George, and Curtis, Wintrrton C. 

igio. Experiments in the artificial propagation of fresh-water mussels. Bulletin, U. S. Bureau of 

Fisheries, Vol. XXVIII, igoS, pp. 615-626. Washington. 
1910a. Reproduction and parasitism in the LTnionidoe. Journal of Experimental Zoology, Vol. 

IX, No. I, pp. 79-115, I fig., 5 pis. Baltimore, 
igii. Metamorphosis without parasitism in the Unionidae. Science, Vol. XXXIII, pp. 863-865. 

New York. 
igi2. Studies on the reproduction and artificial propagation of fresh- water mussels. Bulletin, 
U. S. Btireau of Fisheries, Vol. XXX, 1910, pp. 105-201, pis. VI-XVII. Washington. 
Leidy, Joseph. 

1904. Researches in helminthology and parasitology, by Joseph Leidy, M. D., LL. D. (Arranged 
and edited by Joseph Leidy, jr.) Smithsonian Miscellaneous Collections, Vol. XLVI, 
Art. Ill, pp. 1-281. Washington. 
Maury, C. J. 

1916. Fresh-water shells from central and western New York. The Nautilus, Vol. XXX, pp. 28-33. 

Boston. 
Moore, J. P. 

igi2. Classification of the leeches of Minnesota. Geological and Natural Historj' Sur\ey of Min- 
nesota, Zoological Series No. V, Part III, pp. 65-128. Minneapolis. 
MuTTKowski, R. a. 

igi8. The fauna of Lake Mendota. Transactions, Wisconsin Academy of Sciences, Arts, and 
Letters, Vol. XIX, Part I, pp. 374-482. Madison. 
NEEdham, James G., and Lloyd, J. T. 

igi6. The life of inland waters. 438 pages. Comstock Publishing Company, Ithaca. 
Nelson, Thurlow C. 

igi8. On the origin, nature, and function of the crystalline style of Lamellibranchs. Journal of 
Morphology, Vol. XXXI, No. i, pp. 53-111. Philadelphia. 
New York (City). Report of the Metropolitan Sewerage Commission of New York. 

igi2. Present sanitary condition of New York Harbor and the degree of cleanness which is neces- 
sary and sufficient for water. 457 pages. (Adeney, p. 80.) New York. 
Olt, a. 

1893. Lebensweise imd Entwicklung des Bitterlings. Zeitschrift fiir wissenschaftliche Zoologie. 
Band 55, pp. 543-S7S (i pl)- Leipzig. 
Ortmann, a. E. 

igog. The breeding season of Unionidse in Pennsylvania. The Nautilus, Vol. XXII, No. g, pp. 

91-95, and No. 10, pp. 99-103. Boston, 
igii. A monograph of the Najades of Pennsylvania. Memoirs, Carnegie Museum, Vol. IV, No. 6, 
pp. 279-347, pis. 86-89. Pittsburgh. 

1912. Notes upon the families and genera of the Najades. Annals, Carnegie Museum, Vol. VIII, 

No. 2, pp. 222-365. Pittsburgh. 

1913. The AUeghenian divide, and its influence upon the fresh-water fauna. Proceedings, Ameri- 

can Philosophical Society, Vol. LII, No. 210, pp. 287-390, pis. XII-XIV. Philadelphia. 



l8o BULLETIN OF THE BUREAU OF FISHERIES. 

OrImann, a. E. — Continued. 

1919. A monograph of the Naiades of Pennsylvania. Part III. Systematic account of the genera 

and species. Memoirs, Carnegie Museum, Vol. VIII, No. i, 384 pp., 21 pis. Pittsburgh. 

1920. Correlation of shape and station in fresh-water mussels (Naiades). Proceedings, American 

Philosophical Society, 1920, Vol. LIX, pp. 269-312. Philadelphia. 
OsBORN, Henry LesuE. 

1S98. Observations on the anatomy of a species of Platyaspis found parasitic on the Unionidae of 

Lake Chautauqua. Zoological Bulletin, Vol. II, No. 2, pp. 55-67. Boston. 
Pfund, a. H. 

1917. The colors of mother of pearl. Journal of the Franklin Institute, April, 1917, pp. 453-464. 

Illustrated. Philadelphia. 
REighard, J. E. 

1894. A biological examination of Lake St. Clair. Bulletin, Michigan Fish Commission, No. 4, 
pp. 1-60. Lansing. 
Reuling, F. H. 

1919. Acquired immunity to an animal parasite. Journal of Infectious Diseases, Vol. XXIV, 
No. 4, pp. 337-346. Chicago. 

SCAMMON, R. E. 

1906. The Unionidae of Kansas. Part I. The Kansas University Science Bulletin, Vol. Ill, 
No. 9 (whole series. Vol. XIII, No. 9), pp. 279-373, pis. 62-85. Lawrence. 
Shelford, Victor E. 

1913. Animal communities in temperate America. Bulletin 5, Geographic Society of Chicago, 
362 pages, 306 figs. Chicago. 

1918. Conditions of existence. Ward and Whipple's Fresh-water Biology, Chapter II, pp. 21-60. 

New York. 

1919. Fortunes in wastes and fortunes in fish. The Scientific Monthly, August, 1919, pp. 97-124. 

New York. 
Shira, Austin F. 

1913. The mussel fisheries of Caddo Lake and the Cypress and Sulphur Rivers of Texas and Louisi- 

ana. U. S. Bureau of Fisheries Economic Circular No. 6, lo pages. Washington. 
SiiiPSON, Chari.es Torrev. 

1899. The pearly fresh-water mussels of the United States; their habits, enemies, and diseases, 
mth suggestions for their protection. Bulletin, U. S. Bureau of Fisheries, Vol. XVIII, 
1898, pp. 279-288. Washington. 

igoo. Synopsis of the Naiades, or pearly fresh-water mussels. Proceedings, U. S. National Mu- 
seum, Vol. XXII, No. 1205, pp. 501-1004, PI. XVIII. Washington. 

1914. A descriptive catalogue of the Naiades, or pearly fresh-water mussels. In 3 parts, 1540 pp. 

Bryant Walker, Detroit, Mich. [Ann Arbor Press, Ann Arbor, Mich.] 
SlERKI, V. 

1891. A byssus in Unio. The Nautilus, Vol. V, pp. 73-74. Philadelphia. 
1891a. On the byssus of Unionida:, II. Idem., pp. 90-91. 

1892. A few observations concerning death of fresh-water MoUusca. Idem., pp. 135-136. 
Strode, W. S. 

i8gi. Destruction of Anodonta corpulenia Cpr. at Thompson's Lake, Illinois. The Nautilus, Vol. 
V, pp. 89-90. Philadelphia. 

SURBER, ThADDEUS. 

1912. Identification of the glochidiu of fresh-water mussels. U. S. Bureau of Fisheries Document 

771, 10 pp., 3 pis. Washington. 

1913. Notes on the natural hosts of fresh-water mussels. Bulletin, U. S. Bureau of Fisheries, 

Vol. XXXII, 1912, pp. 101-116, pis. XXIX-XXXI. Washington. 

1915. Identification of the glochidia of fresh-water mussels. Appendix V, Report U. S. Com- 

missioner of Fisheries, 1914, 9 pages, i pi. Washington. 
Utterback, W. I. 

1916. Breeding record of Missouri mussels. The Nautilus, Vol. XXX, No. 2, pp. 13-21. Boston. 



KRESH- WATER MUSSELS. l8l 

Walker, Bryant. 

1913. The Unione fauna of the Great Lakes. Reprinted from The Nautilus, Vol. XXVII (June, 

July, August, and September), 20 pages, 5 figs. Boston. 
1918. Asynopsis of the classification of the fresh-water MoIIuscaof North America, north of Mexico, 
and a catalogue of the more recently described species, with notes. University of Michigan 
Museum of Zoology, Miscellaneous Publications No. 6, pts. I and II, 213 pages, 233 figs. 
Ann Arbor. 
Ward, Henry B. 

1896. A biological examination of Lake Michigan in the Traverse Bay Region. Bulletin, Michigan 
Fish Commission, No. 6, pp. 1-71. Lansing. 
Ward, Henry B., and Whipple, George C. 

igiS. Fresh-water biology, first edition, mi pages. John Wiley Sons (Inc.). New York. 
Wenrick, D. H. 

1916. Notes on the reaction of bivalve mollusks to changes in light intensity: Image formation in 
Pecten. Journal of Animal Behavior, Vol. 6, No. 4, pp. 297-418. Boston. 
Wilson, Charles Branch. 

1916. Copepod parasites of fresh-water fishes and their economic relations to mussel glochidia. 
Bulletin, U. S. Bureau of Fisheries, Vol. XXXIV, 1914, pp. 331-374, pis. LX-LXXIV. 
Washington. 
Wilson, Charles B., and Clark, H. Walton. 

1912. The mussel fauna of the Kankakee Basin. U. S. Bureau of Fisheries Document 758, 52 
pages, 2 figs., i pi. Washington. 

1914. The mussels of the Cumberland River and its tributaries. U. S. Bureau of Fisheries Docu- 

ment 781, 63 pages, I pi. Washington. 
Wilson, Charles Branch, and Danglade, Ernest. 

1914. The mussel fauna of central and northern Minnesota. Appendix V, Report, U. S. Com- 
missioner of Fisheries, 1913, 26 pages, i map. Washington. 
Wolcott, Robert H. 

1899. On the North American species of the genus Atax (Kabr.) Bruz. Transactions, American 
Microscopical Society, Vol. XX, pp. 193-258. Buffalo. 



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